The next time you’re listening to your favorite song or piece of music, try listening for the pauses within it. Those pauses are musical rests, and they’re just as important to the music as the musical notes are. You can see rests in sheet music, too. From wikipedia:
Rests are intervals of silence in pieces of music, marked by symbols indicating the length of the pause. Each rest symbol and name corresponds with a particular note value, indicating how long the silence should last, generally as a multiplier of a measure or whole note.
They include corresponding curriculum (pdf).