Doppler Effect
The Doppler effect is the change in pitch you hear when a sound source moves toward you or away from you.
Think of it this way
Someone stands across the room clapping their hands once per second. As they walk toward you, each clap has less distance to travel. The claps seem to hit your ears faster and closer together. As they walk away, the claps spread out and seem slower.
Explaining doppler effect by grade level
Think about a car honking as it drives past you. The horn sounds higher as the car comes closer. Then the sound drops lower as the car moves away. The sound itself stays the same, but the motion changes what you hear.