It represents the masking of the disposition and movements of enemy forces on radar or spy plane satellite images.
Fog of war is a military term, a common concept. This is simulated, in the radar representation of the battlefield, by a black fog covering the unknown parts of the battlefield and as new visual information is discovered, it diminishes in favor of this new information. This fact causes any commander to be forced to rely on imperfect and incomplete information about the area, and must often improvise based on intuition and common sense.
Normally, as a battle progresses, the entire map area is progressively revealed as a commander’s units “see” the area with their own eyes. As military units learn about their opponents, this information is in turn revealed to the commander, so that he can act. In many cases, this new knowledge is temporary. When units leave the area, their real-time knowledge of the situation on the ground also disappears.