Frequency, in sound, describes how quickly a sound wave vibrates and determines whether a sound is perceived as low or high in pitch.
Frequency is measured as vibrations per second, using Hertz (Hz), where one wave per second is 1Hz.
The range an average human can hear is roughly in the range of 20-20,000 (20k) Hz.
Knowledge of frequency and the limits of human perception are helpful tools in both audio production and audio engineering.