Hertz is a unit of frequency (of change in state or cycle in a sound wave, alternating current, or other cyclical waveform) of one cycle per second. It replaces the earlier term of "cycle per second (cps)."
For example, in the United States, common house electrical supply is at 60 hertz (meaning the current changes direction or polarity 120 times, or 60 cycles, a second). (In Europe, line frequency is 50 hertz, or 50 cycles per second.) Broadcast transmission is at much higher frequency rates, usually expressed in kilohertz (kHz) or megahertz (MHz).
In acoustic sound, the range of human hearing is from 0 Hz to roughly 20 kHz (depending on many factors, including age and how loud the drummer in your high school rock band played!). The pitch of Middle C on a piano is 263 Hz. Hertz is also used frequently when describing the individual bands of an audio equalizer. To make that Middle C louder, you could boost other frequencies to around 263 Hz with an equalizer.
The unit of measure is named after Heinrich Hertz, German physicist.