Srila Prabhupada explains Gundica marjana

ei-mata puradvāra-āge patha yata
sakala śodhila, tāhā ke varṇibe kata
ei-mata—in this way; pura-dvāra—of the gateway of the temple; āge—in front; patha yata—as many avenues; sakala—all; śodhila—were cleansed; tāhā—that; ke varṇibe—who can describe; kata—how much.
In commenting on the cleansing of the Guṇḍicā temple, Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura says that Śrī Caitanya Mahāprabhu, as the world leader, was personally giving instructions on how one should receive Lord Kṛṣṇa, the Supreme Personality of Godhead, within one's cleansed and pacified heart. If one wants to see Kṛṣṇa seated in his heart, he must first cleanse the heart, as prescribed by Śrī Caitanya Mahāprabhu in His Śikṣāṣṭaka: ceto-darpaṇa-mārjanam. In this age, everyone's heart is especially unclean, as confirmed in Śrīmad-Bhāgavatam: hṛdy antaḥ-stho hy abhadrāṇi. To wash away all dirty things accumulated within the heart, Śrī Caitanya Mahāprabhu advised everyone to chant the Hare Kṛṣṇa mantra. The first result will be that the heart is cleansed (ceto-darpaṇa-mārjanam). Similarly, Śrīmad-Bhāgavatam (1.2.17) confirms this statement:
śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ
hṛdy antaḥ-stho hy abhadrāṇi
vidhunoti suhṛt satām
If a devotee at all wants to cleanse his heart, he must chant and hear the glories of the Lord, Śrī Kṛṣṇa (śṛṇvatāṁ sva-kathāḥ kṛṣṇaḥ). This is a simple process. Kṛṣṇa Himself will help cleanse the heart because He is already seated there. Kṛṣṇa wants to continue living within the heart, and the Lord wants to give directions, but one has to keep his heart as clean as Lord Caitanya Mahāprabhu kept the Guṇḍicā temple. The devotee therefore has to cleanse his heart just as the Lord cleansed the Guṇḍicā temple. In this way one can be pacified and enriched in devotional service. If the heart is filled with straw, grains of sand, weeds or dust (in other words, anyābhilāṣa-pūrṇa), one cannot enthrone the Supreme Personality of Godhead there. The heart must be cleansed of all material motives brought about through fruitive work, speculative knowledge, the mystic yoga system and so many other forms of so-called meditation. The heart must be cleansed without ulterior motive. As Śrīla Rūpa Gosvāmī says: anyābhilāṣitā-śūnyaṁ jñāna-karmādy-anāvṛtam. In other words, there should not be any external motive. One should not attempt material upliftment, understanding the Supreme by speculative knowledge, fruitive activity, severe austerity and penance, and so on. All these activities are against the natural growth of spontaneous love of Godhead. As soon as these are present within the heart, the heart should be understood to be unclean and therefore unfit to serve as Kṛṣṇa's sitting place. We cannot perceive the Lord's presence in our hearts unless our hearts are cleansed.
saṁsāra viṣānale, divā-niśi hiyā jvale,
juḍāite nā kainu upāya
Modernized material benefits are like the dust of material contamination. When this dust is agitated by the whirlwind of fruitive activity, it overcomes the heart. Thus the mirror of the heart is covered with dust. There are many desires for performing auspicious and inauspicious activities, but people do not know how life after life they are keeping their hearts unclean. One who cannot give up the desire for fruitive activity is understood to be covered by the dust of material contamination. Karmīs generally think that the interaction of fruitive activities can be counteracted by another karma, or fruitive activity. This is certainly a mistaken conception. If one is deluded by such a conception, he is cheated by his own activity. Such activities have been compared to an elephant's bathing. An elephant may bathe very thoroughly, but as soon as it comes out of the river, it immediately takes some sand from the land and throws it all over its body. If one suffers due to his past fruitive activities, he cannot counteract his suffering by performing auspicious activities. The sufferings of human society cannot be counteracted by material plans. The only way suffering can be mitigated is by Kṛṣṇa consciousness. When one takes to Kṛṣṇa consciousness and engages himself in the devotional service of the Lord-beginning with chanting and hearing the glories of the Lord-the cleansing of the heart begins. When the heart is actually cleansed, one can clearly see the Lord sitting there without any disturbance. In Śrīmad-Bhāgavatam (9.4.68) the Lord confirms that He sits within the heart of the pure devotee: sādhavo hṛdayaṁ mahyaṁ sādhūnāṁ hṛdayaṁ tv aham.
Impersonal speculation, monism (merging into the existence of the Supreme), speculative knowledge, mystic yoga and meditation are all compared to grains of sand. They simply cause irritation to the heart. No one can satisfy the Supreme Personality of Godhead by such activities, nor do we give the Lord a chance to sit in our hearts peacefully. Rather, the Lord is simply disturbed by them. Sometimes yogīs and jñānīs in the beginning take to the chanting of the Hare Kṛṣṇa mahā-mantra as a way to begin their various practices. But when they falsely think that they have attained release from the bondage of material existence, they give up chanting. They do not consider that the ultimate goal is the form of the Lord or the name of the Lord. Such unfortunate creatures are never favored by the Supreme Personality of Godhead, for they do not know what devotional service is. Lord Kṛṣṇa describes them in the Bhagavad-gītā in this way:
tān ahaṁ dviṣataḥ krūrān
saṁsāreṣu narādhamān
kṣipāmy ajasram aśubhān
āsurīṣv eva yoniṣu

By His practical example, Śrī Caitanya Mahāprabhu has shown us that all the grains of sand must be picked up thoroughly and thrown outside. Śrī Caitanya Mahāprabhu also cleansed the outside of the temple, fearing that the grains of sand would again come within. 
In this connection Śrīla Bhaktisiddhānta Sarasvatī Ṭhākura explains that even though one may become free from the desire for fruitive activity, sometimes the subtle desire for fruitive activity again comes into being within the heart. One often thinks of conducting business to improve devotional activity. But the contamination is so strong that it may later develop into misunderstanding, described as kuṭi-nāṭi (faultfinding) and pratiṣṭhāśā (the desire for name and fame and for high position), jīva-hiṁsā (envy of other living entities), niṣiddhācāra (accepting things forbidden in the śāstra), kāma (desire for material gain) and pūjā (hankering for popularity). The word kuṭi-nāṭi means "duplicity." As an example of pratiṣṭhāśā, one may attempt to imitate Śrīla Haridāsa Ṭhākura by living in a solitary place. One's real desire may be for name and fame-in other words, one thinks that fools will accept one to be as good as Haridāsa Ṭhākura just because one lives in a solitary place. These are all material desires. A neophyte devotee is certain to be attacked by other material desires as well, namely desires for women and money. In this way the heart is again filled with dirty things and becomes harder and harder, like that of a materialist. Gradually one desires to become a reputed devotee or an avatāra (incarnation).
To give us practical instructions, Lord Śrī Caitanya Mahāprabhu cleansed the temple twice. His second cleansing was more thorough. The idea was to throw away all the stumbling blocks on the path of devotional service. He cleansed the temple with firm conviction, as is evident from His using His own personal garments for cleaning. Śrī Caitanya Mahāprabhu wanted to see personally that the temple was thoroughly cleansed to the standard of clean marble. Clean marble gives a cooling effect. Devotional service means attaining peace from all disturbances caused by material contamination. In other words, it is the process by which the mind is cooled. The mind can be peaceful and thoroughly cleansed when one no longer desires anything but devotional service.
Even though all dirty things may be cleansed away, sometimes subtle desires remain in the mind for impersonalism, monism, success and the four principles of religious activity (dharma, artha, kāma and mokṣa). All these are like spots on clean cloth. Śrī Caitanya Mahāprabhu also wanted to cleanse all these away.
By His practical activity, Śrī Caitanya Mahāprabhu informed us how to cleanse our hearts. Once the heart is cleansed, we should invite Lord Śrī Kṛṣṇa to sit down, and we should observe the festival by distributing prasāda and chanting the Hare Kṛṣṇa mahā-mantra. Śrī Caitanya Mahāprabhu used to teach every devotee by His personal behavior. Everyone who spreads the cult of Śrī Caitanya Mahāprabhu accepts a similar responsibility. The Lord was personally chastising and praising individuals in the course of the cleaning, and those who are engaged as ācāryas must learn from Śrī Caitanya Mahāprabhu how to train devotees by personal example. The Lord was very much pleased with those who could cleanse the temple by taking out undesirable things accumulated within. This is called anartha-nivṛtti, cleansing the heart of all unwanted things. Thus the cleansing of the Guṇḍicā-mandira was conducted by Śrī Caitanya Mahāprabhu to let us know how the heart should be cleansed and soothed to receive Lord Śrī Kṛṣṇa and enable Him to sit within the heart without disturbance.
Srila Prabhupāda: There were not many. The Rathayātrā is very nice. He was chanting with a group of devotees, and while the car festival is going on. And sometimes car used to stop. It still stops. That is the fashion of Jagannātha. And nobody could, even an elephant could not draw it. Then Caitanya Mahāprabhu will say, "All right, come on." So He would push it with His head and it will go on.
Devotee: What did the people say when they saw that?
Srila Prabhupāda: Then… That was… Caitanya Mahāprabhu already adored, He was. So, there is… Rathayātrā car festival, and Guṇḍicā-mārjana. Guṇḍicā-mārjana. Before the Rathayātrā festival, the system is Jagannātha goes from the temple to another place about two miles away. There is a big temple there also which is called Jagannātha Aunt's(?) house, Mahiṣī badi. So He stays there for eight days. Just like we make program in San Francisco to reside on seaside. So that temple was to be washed. Caitanya Mahāprabhu used to do it by His personal supervision. So, all the devotees… And He will check how much dust you have recovered, and then you shall be rewarded. Otherwise, it should be understood that you have not labored nicely. (chuckles) So He'll sweep over the whole temple nicely and wash the ceilings and walls and everything so clearly. And He'll see in this way if there is any sand. If there is, "Oh, it is not washed." That means He would engage everyone. Then after washing, then call for prasādam and distribute Himself. This is called Guṇḍicā-mārjana. Actually, whatever He was doing, they were washing, "Hare Kṛṣṇa Hare Kṛṣṇa," chanting. That's all.

Devotee: He used to eat a lot of food?

Srila Prabhupāda: Yes. Not always. But people used to present some foodstuffs. Especially when the devotees would come from Bengal side, somebody is bringing something, somebody is… Whatever Caitanya Mahāprabhu likes. And they will prepare the whole year nice foodstuffs. That is, what is called, preserved food. You can keep it for days together. So His personal assistant was Govinda, and everyone will, I mean to say, offer Govinda, "Please offer this food to Prabhu." And Govinda will keep. And everyone is anxious whether his goods are taken. So he was inquiring Govinda, "Has Mahāprabhu has taken my food?" What can he say? "Yes, yes, yes, yes." But it is stacked in the store. So one day Caitanya Mahāprabhu He said that… He was Godbrother also, Caitanya Mahāprabhu. He could talk with Him freely. He was not disciple. So "Guru Mahārāja sent me to serve You, and now the result is that for You I have to speak only lies." "What is that? You are speaking lies for Me?" "Yes. Why not? What can I do?" "Now, what is the matter?" "Now, Your devotees give me so many things for eating and just lying stacked. And they inquire and I say 'Yes, yes. He has taken.' So, this is my business, telling lie." So, then Caitanya Mahāprabhu said, "All right. Bring something. I shall eat." So, he brought one bag and He ate everything. "Bring next." In this way the whole stack, about 100 men's foodstuff, He ate. Then He asked him, "Bring more." "Now the bags are lying only. If You want to eat, (chuckling) You can eat." "All right. Stop." So in half an hour, one hour, He ate all the 100 men's stock. You see. This is also another miracle. He finished the whole stock to save him from speaking lies. 
But in the modern age this Kṛṣṇa consciousness movement, Hare Kṛṣṇa movement, was started by Lord Caitanya, and this Ratha-yātrā is part of this movement. Therefore we have introduced this Ratha-yātrā festival in our society. And the boys and the girls, they are taking it very nicely, and it will go on. So the Lord Caitanya was taking the part of Rādhārāṇī, when Rādhārāṇī requested Kṛṣṇa to come to Vṛndāvana. So when Lord Caitanya was before the Ratha-yātrā, He was thinking that "I am taking Kṛṣṇa to Vṛndāvana." That is His ecstasy. "I am taking Kṛṣṇa to Vṛndāvana."

So that function is observed yearly, and Kṛṣṇa, Jagannātha, goes to that Gundica. So today is Guṇḍicā-mārjana day. So Lord Caitanya personally used to wash the whole temple along with His assistants. Hundreds of… At that time there was no hose pipe, but people used to bring water in big, big waterpots. So when Lord Caitanya wanted to wash the Guṇḍīca Mandir, the king of Jagannātha Purī, Mahārāja Pratāparudra… He was a great devotee of Lord Caitanya. So it was his open order to his officers that whenever Caitanya Mahāprabhu will ask for anything, it must be immediately supplied. So during this time about… In those days there was no railway facilities five hundred years ago. So people used to go on foot from, especially from Bengal. And because He was staying in Orissa, there were many Oriyan-Gauḍīya. Practically Lord Caitanya's followers are the Bengalis and the Oriyas. They are generally. Because Lord Caitanya appeared in Bengal and resided, He made His residence in Orissa. Naturally, the people of these two provinces became His devotees in large number. And still, in Orissa there are many devotees, as there are in Bengal. So Caitanya Mahāprabhu was patronized or… Mahārāja Pratāparudra was very powerful king. He did not allow the Mohammedans to enter into Orissa. He was so powerful. At that time whole India was occupied by the Pathans, but they could not enter in Orissa and South India. So Mahārāja Pratāparudra was very powerful king, and at the same time he was a devotee of Lord Caitanya. So his open order was that whenever and whatever Caitanya Mahāprabhu will ask anything, it must be supplied. So on this day Caitanya Mahāprabhu used to wash that Guṇḍīca Mandir with hundreds of His followers. And they were ordered to bring water from the nearest tank. There is one tank. If you go sometimes to Jagannātha Purī, you'll see that place. So, from that tank hundreds of waterpots were brought, and first of all it was… What is called? Sweeping. The sweeping process is first of all taken. And He wanted to see all the devotees, "How much dust you have gathered." He'll see personally. "Let Me see what is the amount of your dust you have gathered by sweep… [break] Then I will understand that you worked very hard.” So then after it is very finely, twice. First of all, once swept, then second time. Not even a small particle grain should remain. That was His order. [break]. 
ei-mata mahāprabhu lañā nija-gaṇa
guṇḍicā-gṛhera kailā kṣālana, mārjana
ei-mata—in this way; mahāprabhu—Śrī Caitanya Mahāprabhu; lañā nija-gaṇa—accompanied by His personal associates; guṇḍicā-gṛhera—of the Guṇḍicā temple; kailā—performed; kṣālana—washing; mārjana—cleansing.
Accompanied by His personal associates, Śrī Caitanya Mahāprabhu washed and swept the Guṇḍicā temple, cleansing it as usual.  

pūrvavat sabā lañā guṇḍicā-mārjana
ratha-āge pūrvavat karilā nartana
pūrva-vat—as previously; sabā—all the devotees; lañā—taking; guṇḍicā-mārjana—the cleaning of the Guṇḍicā temple; ratha-āge—in front of the chariot; pūrva-vat—as previously; karilā nartana—danced.
All the devotees engaged in the cleansing ceremony of the Guṇḍicā temple and danced in front of the Ratha-yātrā chariot, just as they had done in the past.