Official Aviation Definition
Air Traffic Control (ATC) is a service provided by ground-based personnel who direct aircraft on the ground and through controlled airspace, and provide advisory services to aircraft in non-controlled airspace. The primary purpose of ATC worldwide is to prevent collisions, organize and expedite the flow of air traffic, and provide information and other support for pilots.
In Simple Words
Think of Air Traffic Control as the traffic police and dispatchers of the sky. They tell pilots when to take off, which altitude and route to fly, and when it is safe to land so planes never hit each other.
Real-World Flight Example
Before pushback at London Heathrow, the flight crew contacts Ground Control (a branch of ATC) to receive taxi instructions to Runway 27R.