What is the Doppler Effect?

The Doppler Effect is the strectching and compressing of waves caused by the movement of the source. Put more simply, when an object emits a sound - say a van beeping its horn it produces a sound of a certain frequency. If the van was moving while beeping its horn then the frequency of the sound will be changed relative to an observer (someone watching the van from the pavement). If the van is moving towards the observer then the relative frequency is increased so the pitch of the sound is also increased, while if it's moving away the frequency is decreased. This is caused by the sound waves bunching up in front of the van and stretching out behind the van as each successive wave front is produced at a certain time later, so the van has moved a certain distance and the next wave to be produced is that extra distance closer or further away from the previous wave. Therefore, the waves appear stretched behind and compressed in front of the moving van. 

This can be given in the Doppler Equation:

f' = f( v /v-v_s) 

where f is the frequency of the horn; f' is the observed frequency; v is the speed of sound in air and v_s is the velocity of the van relative to the observer. 

The Doppler Effect isn't just limited to Sound but can be applied to all waves which means it can give us deep insights about Light, allowing us to have meaningful revelations about the Expanding Universe.