Early historical maps were usually one-of-a kind unique hand-drawn creations. Even if the map were published, the additional copies of the map were duplicates or near duplicates of the original. As a result of the individuality of the maps, there was no easy way to compare places shown on one map to those same places shown on another map. The problem was the lack of a uniform scale.
In maps, the term "scale" refers to the relationship between a measured distance on the map and the measured distance on the actual portion of the earth being represented. If the scale were 1 to 1 (usually represented by 1:1), the map would be the same size as the land or building represented. For very small objects a 1:1 representation is reasonable and practical. For land, showing things at full size is usually very impractical. Any map drawn to a particular scale will only be entirely useful if the map also shows the scale represented.
Here is a screen shot of the bottom of a topographical map showing the scale:
The bar scale shown in the screen shot above, gives you a measuring size so that you can measure almost exact distances on the map. The bars are calibrated in meters, miles and feet. Remember that a mile is 5280 feet.
Many other map scales have been used historically and many are in use today. Some of the most common scales for topographical maps include the 1:62,500 or a 15 minute quadrangle, the 1:100,000 or one degree sheet and the 1:250,000 or 2 degree sheet. The reference to "minute" is the measurement on a circle i.e. curved surface of the earth's sphere in degrees, minutes and seconds. (360 degrees in a circle, 60 minutes in a degree, 60 seconds in a minute). The smaller the scale of the map, such as 1:250,000 the larger the area on the actual earth surface, the map covers. Large scale maps cover a small actual world area. Small scale maps cover a large actual world area.
Now you are asking, who cares and why should I know this? A map is not just a picture, it is supposed to be a representation of a portion (or all) of the earth's surface. Old historical maps were sometimes drawn with a variable scale. That is, the scale varied from one location to another. Places well known to the map maker were drawn with greater detail than those unknown or at some distance. It is often very difficult to try and match an historical map to its modern equivalent for this reason alone. If you are aware of the scale of a map every time you look at one, you will not be mislead by places that appear to be quite close but, due to the scale of the map, may actually be hundreds of miles apart. A map is only accurate if it has a consistent and measurable scale.