17.23 Here Brahman means the Veda. It is the secondary meaning of the expression, especially of the ritualistic portion of the Veda. The three-fold expression Om Tat Sat is connected with the Brahman  or the Veda. Sacrifices and similar rituals are prescribed in the Vedas. These expressions Om Tat Sat are used in these Vedic rites. The connection of Om is that it should be invariably used at the commencement of the recitation of Vedic hymns. The syllable Tat and Sat indicate that these rituals are worthy of honour. The Brahmanas are those who are to preserve Vedic study as also the Vedas and the sacrificial rites ordained in them. All these were created by Me in the past.

Sri Krsna elaborates in the next verses the nature of the connection of these syllables with the Vedic rituals, beginning first with Om.