Glossary
Map control
In one line: The areas of the map where your team can move without being seen or shot.
What it means
Having map control means "owning" areas: you know there is no enemy there, or that you will see them if they come. On attack, the more control you have, the more options you have on where to go; on defense, control lets you see the attack coming and rotate in time.
Control is taken (with utility, players, information) and lost (when nobody watches that area anymore). It is not a goal in itself: it answers a question, such as "can I go B through mid without being seen?".
An example
Your initiator clears mid with a drone and two teammates settle there. Now you have mid control: you can hit A or B through mid, and the defense cannot know which one you will pick.