Explain what is meant by the Doppler Effect. Explain one real-world application of the Doppler Effect

An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move toward (or away from) each other

an example, would be hearing a fire truck's sirens in the distance, and then as it gets closer, and passes you, the sound of the sirens gets louder, and as it gets farther away, the sound becomes more faint