Answer:
I assume you're talking about Doppler Weather Radar. It measures reflectivity over long distances. These radars scan the atmosphere at different levels and when the radar sees an object in the atmosphere (such as rain, bugs, etc) it plots the location and gives a measurement of how reflective the object is. Doppler Weather Radar is very useful for showing where it's currently raining, snowing or hailing---and how heavy it is. It can also track the movement of precipitation and other wind-driven particles in the air which allows it to calculate the speed of wind and if the wind is blowing toward or away from the radar tower. Seeing the wind directions and speeds inside of a thunderstorm help meteorologists determine areas of rotation in storms (which may mean that a tornado is developing)...