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).