Answer:
If you have a source that emits a wave and an observer that is motionless with respect to the source, no Doppler Effect is noticed. If the source and observer move with relation to each other then the wave appears distorted (compressed if the motion is towards each other, distended if the motion is away from each other). Say we have a source of sound by the side of the road (a repeatedly clanging bell). If an observer moves towards it, the sound waves appear to meet the observer sooner and the pitch of the bell (and frequency of the clanging) increase. After passing the bell and while moving away, the frequency of the clanging and the frequency of the sound drop.