Fog of war
Fog of war is the idea that in war you have little knowledge of enemy actions that occur outside your line of sight.
In Empire Earth this is portrayed in that any place on the map that no friendly units have visited is obscured (black), so you can't see any units, buildings, or terrain in that area. When a friendly unit visits a location, the area within its line of sight comes in to full view (brightly lit), and you are able to see everything in the area. When the friendly unit(s) are no longer in that area, the area becomes darker (shaded) and you can only see the terrain and buildings that were there at the time the unit(s) were there.
When diplomacy is involved, players who are allied with each other share explored areas of the map and line of sight.