The frequency of a sound wave is interpreted by our ears as pitch, and the frequency of a light wave is interpreted by our eyes as color. If the wave source is moving, it can cause a difference between the actual frequency of the wave being emitted and the frequency that is received by our eyes or our ears. This image shows a wave source moving to the left. Notice that the waves on the left-hand side are bunched closer together while the waves on the right get stretched out.

Source: Doppler Effect diagrammatic, Antilived Wikimedia Commons

Watch the following video for a step-by-step description of how this works.