We can see objects in the world around us because light (usually from the Sun) reflects off them into our eyes. If you want to walk at night, you can shine a torch in front to see where you're going. The light beam travels out from the torch, reflects off objects in front of you, and bounces back into your eyes. Your brain instantly computes what this means: it tells you how far away objects are and makes your body move so you don't trip over things. Radar works in much the same way. The word "radar" stands for radio detection and ranging—and that gives a pretty big clue as to what it does and how it works. Imagine an airplane flying at night through thick fog. The pilots can't see where they're going, so they use the radar to help them.
An airplane's radar is a bit like a torch that uses radio waves instead of light. The plane transmits an intermittent radar beam (so it sends a signal only part of the time) and, for the rest of the time, "listens" out for any reflections of that beam from nearby objects. If reflections are detected, the plane knows something is nearby—and it can use the time taken for the reflections to arrive to figure out how far away it is. In other words, radar is a bit like the echolocation system that "blind" bats use to see and fly in the dark.