Map control is a very difficult term to fully describe. However, the general idea is that the player who has map control has the majority of vision on the map and the army strength necessary to move out past the half-way point between your two main bases. So how do you get map control?
First off, building a mobile army is an effective way to take map control. Units like Hellions, Speedlings, and Blink Stalkers let you dart in and out around the map – helping increase your vision of what is going on as well as assert your presence on the map.
Second off, taking the Xel’Naga Watchtowers is an easy way to take map control. They provide a lot of vision and initially only require a small army commitment in order to claim them. Fight for the watchtowers in the late game too – for map control does not end after the early game, and seeing your opponent’s army moving out before it gets to your base can be crucial.
Thirdly, scouting around the map with cheap units is a great way to take map control. A Zergling at a watchtower is good, but what if a Probe sneaks past it and sets up a proxy Pylon where you weren’t expecting. Use Observers to patrol common drop paths so you can intercept them before they land. Send Marines to the corners of the map in order to spot army or Medivac movement.
Whichever race you play, be sure to try and take map control in whatever way possible. Map vision and army mobility can earn you a lot of free wins. Losing it can cost you a lot of tough losses.