I can think of one analogy.
Think of a stone which is dropped in a pond. It creates circular ripples (like sound waves) which radiate from the point at which the stone is dropped.
If there is a non-moving boat in the water, the ripples will reach the boat at normal frequency.
If the boat is moving towards the stone, the ripples will reach it at higher frequency.
If the boat is moving away from the stone, the ripples will reach it at lower frequency.
In other words, when the boat is moving, there is disturbances in the spacing of the ripples.
