Radar is use to detect where something is, where it’s going and how fast it’s moving.
Radar has a transmitter that produces radio waves and a receiver that detects them. The system first sends out radio waves. The waves hit an object and some get reflected back toward the radar. As they return, the waves are picked up by the radar’s receiver. The time that it takes for the waves to come back and how the waves have changed when they return provides useful information about the object ahead.
This is a radar antenna on top of a boat. It emits a rotating beam of signals to help detect other boats (and avoid hitting them).