Map scale is the relationship between a unit of length on a map and the corresponding length on the ground. You can find an explanation of the different types of scales you will see on maps at http://www.sfei.org/it/gis/map_interpretation/maps-and-scales.
Half-way down that page is a section on finding area measurement from scale, etc. This is useful if you need to figure out what area on a paper map represents what area on the surface of the earth. For example, you might need to figure out how many square inches of the map you are looking at represents 50 square miles on the surface of the earth.
You can also also Google "map scale converters" to find some handy online tools.
The Knight Reference desk has a few rulers available if you need to measure distances on a map.