Fog of War is an element of gameplay mostly found in strategy games, from Starcraft to Fire Emblem. It is a darker, fog-like layer that hides everything but the terrain and buildings. It disappears as units approach the fog, depending on their vision range.
Fog of War is meant to make strategy games harder. There's an element of mystery and suspicion when you don't know where the enemy is, or what lies ahead of the path. Games handle Fog of War differently. In Advance Wars, the Fog only hides units that are not in range, not terrain. In Age of Empires II, Fog will hide everything, until you explore it.