A map display is a graphical display of position information and other data presented overlayed on a map.
Imagine having a nice topographical map or street-level map and being able to mark an 'X' on it: I am here. Then that map would be instantly shared with everyone who might be interested, via RF and the APRS-IS network. Of course, not everyone will see the same thing because only the position information is shared. Some people may not have that map. Or their map may be faulty in some way.
Position reports are called posits. They can have a built in estimate of ambiguity. Position reports can be created using the GPS system, by manually entering coordinates, or by interactive selection on the map display.