Imagine this: You're standing in a crosswalk waiting to cross the street. While you wait, you see a firetruck turn onto the street, lights flashing and sirens blaring. You notice that as the truck gets closer to you, its siren sounds louder and higher until the truck passes you and the siren's sound becomes quieter and lower as the truck disappears down the road.
The pitch of the siren's sound changes based on where it is in relation to you and how fast it's moving -- all while the source of the sound (the siren) and the velocity of the sound waves remain unchanged. As the siren gets closer to you, the pitch of its sound increases (higher and louder) because the closer the siren is to you the shorter the sound waves between you and it. The same is true in reverse: As the siren passes and gains distance from you, the pitch of its sound decreases (quieter and lower) as the sound waves between you and it become longer.
This phenomenon is named for Christian Johann Doppler, the 19th-century physicist who made the discovery.