I don't know the expression...
So, mind I will try my best to tell what Doppler effect is
Suppose an object is emitting waves (light waves, sound waves...). When it is stationary, its waves spread uniformly around it. But, when it is moving, its waves tend to get concentrated on its moving direction, but the waves behind it spread outwards.For, the observer, the object seems to be with high frequency
Here is visual example: