These air traffic controllers are watching the planes on radar. Radar bounces a signal from the ground into the sky, and when it hits a plane, it bounces back. This makes a mark on the radar, showing where the plane is.

So they know which moving dot on the radar is which, there is a secondary form of radar. This sends a message to a transponder in the plane, and the plane sends back a message saying which airplane it is. This makes a little label beside the dot on the radar.

The controllers must watch to make sure none of the dots ever get too close to each other.