Sound is a form of energy. When you clap your hands, slam a door, talk, or pluck the rubber band on your cup-and-rubber-band “voice box,” those actions make air molecules vibrate. One molecule knocks into another, which bumps another, eventually creating a wave.
When a series of these waves push on your eardrum, you hear. The number of times per second that a wave pushes on your eardrum is called the frequency of the sound.