Frequency is the rate of repetition of a regular event. A wave has a repeating pattern. One such repetition is known as a wave cycle. When talking about sound, frequency means the number of cycles of sound in a second.
The following diagram shows a high frequency wave (top) and a low frequency wave (bottom), plotted as pressure versus time. The high frequency wave has completed twelve cycles over the time shown. The low frequency wave has completed only three cycles over the same time.
The unit used to measure frequency is named Hertz, which is defined to be the number of cycles in one second. (This unit is named after Heinrich Hertz, a famous 19th century physicist).