Question

What is the dopplereffect?

Answer 1

The Doppler effect (or Doppler shift) is the change in frequency of a wave (or other periodic event) for an observer moving relative to its source. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer.
 the best example to this is when you throw a stone in water and the waves are created which gradually recede away when moving away from the object thrown.

Answer 2

Doppler effect is the alteration in the frequency of a wave  for an observer moving relative to its source.