Answer:
Take a car for example. When the car is moving to the left, each successive wave is emitted from a position further to the left than the previous wave. So for an observer in front (left) of the car, each wave takes slightly less time to reach him than the previous wave. The waves "bunch together", so the time between arrival of successive wavefronts is reduced, giving them a higher frequency. For an observer in back (right) of the car, each wave takes a slightly longer time to reach him than the previous wave. The waves "stretch apart", so the time between the arrival of successive wavefronts is increased slightly, giving them a lower frequency.