The effect was first described by Austrian physicist Christian Doppler (1803-1853).

It is the apparent difference between frequency at which waves (light, sound and radio) leave a source and that at which they reach an observer. It is caused by the relative motion of the observer and the wave source. It can be observed by listening to the siren of an approaching vehicle-its pitch rises as it approaches the observer and falls as it moves away. It is used in radar and to calculate the speed of stars by observing the change in frequency of their light.

Note:

Our knowledge of the expanding universe can be verified by observing starlight which emits redshifts, which means that they are moving away. If a star or galaxy approaches our planet, we will be able to observe blueshifts, meaning that they are approaching us.