Map control is very important in RTS based gameplay, but can be a very difficult concept to master. What map control boils down too is being able to react quickly to what is happening on the map, and you can't react to what you can't see. Scouting is an integral part in map control. Good scouting can show when the enemy is going for an expo and reveal enemy locations. Being able to react to stuff happening on the battlefield is important as well. Usually having fast units or units that can bypass terrain is very integral to map control. Having zerglings or mutas to deny expansion can set back an opponent, and having really fast units to prevent enemy movement can cause the enemy to turtle up while you lay claim to all the minerals on the map. Xel'Naga towers are also very important, and controlling multiple of them in one game can ensure you will know whats going on.
