The Doppler effect is known after Doppler but even if the name is unfamiliar the effect most certainly is not. A common example is the sound of a train, ambulance or other vehicle coming towards the listener at speed, passing by then continuing away form the listener. Imagine this and you will "hear" the Doppler effect.

The Doppler effect is the apparent change in frequency to an observer when the source of the waves moves relative to the observer. There are many web simulations which explain this as these samples show. They can also be used to show how the sound barrier and supersonic booms arise. The Doppler effect is important not just for sound but in other waveforms. It can be used in many kinds of wave analysis. At school level the most important of these is related to the Big Bang theory of the origin of the universe. Here are some simulations.