A unit of measurement for vibration, one Hz is equal to one cycle per second. It replaces the earlier term "cycle per second (cps)." For example, in the United States, the common household electrical supply is at 60 hertz (meaning that the current changes direction or polarity 120 times, or 60 cycles, per second).
In Europe, line frequency is 50 hertz, or 50 cycles per second. In general, 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 20 Hz to roughly 20,000 Hz (which equals 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 use an equalizer to boost other frequencies to around 263 Hz. This unit of frequency is named after Heinrich Hertz, a German physicist.