A HERTZ is a unit of measurement that denotes anything that happens ONCE in a given period of TIME. Typically, that measurement of time is in seconds.
Webster defines it as:
"A unit of frequency equal to one cycle per second"
WhatIs.com states:
"In acoustic sound, the range of human hearing is from [20 Hz Hertz(sic)] to roughly 20 KHz [Kilohertz] (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."
To put this in perspective:
In normal speech, vocal cords can move over 100 times per second. Eight hertz would represent the vocal cords moving only 8 times per second to produce the record sound!