A radar speed gun is a Doppler radar device, pointing or aiming the radar gun at the moving target, a measurement of speed is achieved by the radar gun. It does this by detecting a change in the frequency of the returned radar signal. This change in frequency is caused by what is known as the Doppler effect.
So for example, the frequency of the returned signal is increased in proportion to the object’s speed of approach if the object is approaching, and the frequency of the return signal is lowered if the object is receding.