The Doppler effect occurs in everyday life more often than you may think. It is the change in frequency or wavelength of a wave relative to a moving observer.
In this animation a car has sound waves that it is emitting. As the car starts to move the wavelength of the wave becomes smaller on the side of the car that it is moving toward and larger on the side that it is moving away from. An observer on the left side of this car would hear it gradually become higher pitched. An observer on the right side of the car would hear it gradually become lower in pitch.
The Doppler effect is true for light waves as well. This is wear the terms red-shift and blue-shift come from. Blue light has a higher frequency than red light; therefore if an object appears to be becoming more blue that means it is moving toward you. If an object appears to be turning more red than it is moving away from you. This is only noticeable on very large scales like when observing space. Because of the Doppler effect, scientists have determined that our universe is expanding since most objects in space are red-shifted relative to earth.