Explain the purpose of a key on a map

It shows what certain symbols or pictures could mean on a map, and in addition, could show a scale that converts inches or centimeters that you can measure with a ruler to miles. 

The purpose of a key on a map is so your map can be organized and you know what certain symbols mean. For example you could have ^ as a symbol for mountains but if you didnt have a key you wouldnt know what ^ meant