A map key is found on a map. It tells what certain symbols on the map mean. For example, the "star" on most maps means state/country capitals. It is in a small box.
A map key
the key of the map. Or the map key. they are the same thing
A map key looks like a map with keys on it !
The part of a map which explains its symbols are called legends/map symbols.
A map key will have the meaning of the different symbols that are found on a map. So if there are tree symbols on the map the key may explain that these symbols mean there is a park in those locations.