Always a part of well-balanced RTS games, Map Control (commonly referred to as MC) is the amount of control you exert on the map. At the simplest level this can refer to the amount of map that you have expanded to cover, to access a greater resource base. However, there can also exist "controlled" territory that has not yet been expanded into - an area covered by an air force, for example. Converting Controlled territory to Expansion territory effectively is one of the cornerstones of effective play.
Map Control is ESSENTIAL in almost all maps/mods, however it cannot be the sole method of achieving victory, considering the economic infeasibility of maintaining it inviolate of assault.
The conundrum is this- if left unchallenged, economic growth is exponential with territory acquisition. The more you expand, the more resource points you control, and the more room you have to build up your on resource or logistics points. However, the more you expand, the more thinly your defenses are spread to protect against raiding. Your opponent also has more choices for targets to mobilize an assault against, making the "guessing game" elements of this strategy more difficult for the expanding player. Expansion alone is not a winning strategy - you must combine expansion with good use of the additional resources, flexible force control that can react quickly and effectively to widespread attacks, and plenty of scouting so you can keep up with where and when your opponent is planning an attack.
A common mistake of the beginner player is to expand too slowly, and be crushed by the superior military machine of his opponent. A common mistake of the intermediate player is to expand too quickly and leave a vital hole in his defenses that the enemy can exploit. Mastering the rate of expansion, which varies from map to map and mod to mod, is quite possibly the most difficult and most rewarding strategic problem a player has to face.