The symbols of a map are explained in its:

The map key explains what they mean. Map keys are often boxes in the corner of the map, and the information they give you is essential to understanding the map. Symbols in the key might be pictures or icons that represent different things on the map

Explanation:

The symbol of a map are explained in its legend or map key.  

Explanation:

Map key provides additional information to better understand the map. Not all information can be included in the actual map, therefore, the map key serves this purpose. It is normally written in a box and positioned at the corner of the map.  

Compressive maps use different colors, have different feature representation, etc. Symbols which are sometimes pictures or icons used for different purposes are further given an explanation on the map key. It sometimes contains scale representation.