Plain English Meaning
ATC is the team of trained controllers on the ground who guide and separate aircraft. They talk to pilots by radio, tell them where they can go, when they can take off or land, and keep aircraft a safe distance apart.
Full Definition
A service operated by the FAA to promote the safe, orderly, and expeditious flow of air traffic. ATC personnel, working from control towers, approach control facilities, and en route centers, issue instructions and clearances to pilots to separate aircraft from each other and from terrain, and to manage the use of airspace and airport surfaces.