The Doppler shift or Doppler effect is the change in frequency of a wave due to its relative motion between the source and receiver. In everyday life, it's commonly heard when a vehicle sounding a siren or horn - such as a police car or ambulance - approaches, passes, and recedes from an observer resulting in a change in pitch of the sound.
In astronomy, the Doppler shift is used to measure distances of remote astronomical objects such as a galaxies or quasars. The term redshift is used for receding objects since the light is shifted towards the red end of the visible spectrum. Alternatively, blueshift is used for approaching objects. The vast majority of galaxies have redshifts due to the expansion of the universe.