Doppler effect is simply, the apparent change in the frequency of a wave(such as light or sound) that we observe whenever the source of the wave is moving relative to the observer.
For example, if you were on the street and a police car or ambulance were passing. At one time you would hear the horn's pitch increasing(when it is approaching you),
and some other time you would here the pitch lowering as the ambulance gets away form you.
You would have observed Doppler effect!
The frequency(and wavelength) only appear to have changed.
If you were moving in the same direction and at the same speed as the ambulance then you would not experience a change in pitch.
Likewise, if both the ambulance and yourself were stationary.
This tells you that: Doppler effect only occurs when there is relative motion!
When calculating the apparent frequency