Radar works by sending out a radio wave at a very high frequency.
When the radio signal hits raindrops part of the signal bounces back to the radar. Knowing exactly how fast the signal is travelling, means that we can tell how far away the rain is by timing how long it takes for the signal to travel to the rain and then bounce back to the radar. Doppler radar is now being used by Meteorologists to learn even more about weather. Especially dangerous weather like tonadoes. Doppler radar can see the raindrops going in different directions and tell a forecaster right where the tornado is in some cases.