Most of the map displays the type of terrain of the respective room on the map (grass fields, mountains, shallow water, roads, etc.). For a full listing of all the terrain types, which are traversable, which are not traversable, and which are area entrances, type help mapshow.
The terrain on the map will not show if certain mobiles or objects are in the room, which you can see. In the picture at the top of the page, your character would be represented by the two bright cyan () in the center of the map. (The map is always shown with you centered.) The purple #@ displays that there are creatures/mobiles in that room, and how many. If it is dark cyan, it represent players, and dark red represent a combination of the two. If the quantity goes into double digits, the @ is overwritten. Also, if there is combat going on in the room then the colors will be bright instead of dark.
If there is a player's corpse on the map, it will show up as X@ in dark grey. If there is a sign post in the room, you will see 'SP' in a dark yellow. Just as these are prioritized higher than showing the terrain type of the room, any creatures/players in the room are prioritized above objects.