If air waves hit your eardrums in rapid succession, your brain interprets this as a high pitched sound (wind chimes, for example). If the waves hit your eardrum less rapidly, your brain interprets this as a low-pitched sound (a foghorn is a good example).
The number of waves-per-second hitting your eardrum is called the "frequency." Sound frequency is measured in units called "hertz." If 100 waves hit your eardrum in one second, that's a 100 hertz sound. (Hertz is abbreviated hz).
Sound hits your eardrum
Imagine the blue line at the right is your eardrum. It vibrates back and forth as sound waves make contact. Your brain interprets these physical vibrations as sound.