Doppler effect also known as Doppler shift, is the change in frequency of a wave for an observer moving relative to the source of the wave. E.g. the siren of a fast approaching train you heard usually much higher than a fast departing train.

The Doppler effect equation is:
  f = f₀ * (v + v_r)/ (v + v_s)

Where:

  v: the velocity of waves in the medium
  v_r: The velocity of the receiver relative to the medium, positive if the receiver is moving towards the source
  v_s: The velocity of the source relative to the medium, positive if the source is moving away from the receiver
  f_s: Emitted Frequency
  f: Received Frequency

The most widely used Doppler effect application is radar, e.g. police radar and radar for air traffic control.