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.
AC current in the US has a frequency of 60 Hz. An FM station at 100 on the FM dial has a carrier frequency of 100 million cycles per second, or 100 megahertz.
The unit is named after Heinrich Hertz, German physicist.