The Doppler effect is an observed shift in the frequency of a wave as the source of the emission moves relative to the observer. As a sound- or light-emitting object moves closer to an observer, the pitch of the emissions rises. As the object recedes, the pitch falls.
The Doppler effect is easily observed in the rising and falling of traffic sounds. As a car approaches an observer, the sound waves emitted by its horn pile on each other and reach the observer in rapid succession. As the vehicle passes, the reverse of this process stretches out the distance between wave crests and lowers the frequency with which they reach the observer, generating a dropping pitch.