Hertz are the amount of cycles or refreshes a second.
It is the amount of frames the screen is displaying a second.If you have a LCD then set it to 60. If you have a CRT set it to the highest your monitor supports.
Hertz = cycles per second
Megahertz = 1,000's cycles per second
Gigahertz = 1,000,000's cycles per second
Terahertz = 1,000,000,000's cycles per second