The Doppler Effect is illustrated by a car driving by you on the highway. As the car approaches you, the sound is of a certain pitch. When passes you, the sound changes (drops).

The reason for this is because when the car is approaching you, the frequency of the sound wave reaching your ears is increased by the speed of the car driving towards you. When the car is going away from you, the frequency of the sound wave reaching your ear is reduced by the speed of the car driving away from you.

That's the Doppler effect.

Now imagine a cow floating 6" above the road, going 112MPH wearing aviation goggles and a scarf... mooing.

That's dopplercow.