The Doppler Effect is used to determine the size of the universe as well as predict the weather. Watch this video to get an idea of just what is happening when we experience this scientific phenomenon.
Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - an apparent shift in frequency for a sound wave produced by a moving source.