Hertz (Hz) is the measure of the number of Cycles that a wave makes in one second...ex. 1 Hz=1 cycle per second, 100 Hz=100 cycles per second. Human hearing streches from about ~20 Hz-20,000 Hz. Lower frequecies (80Hz-and down) are generally the ones in an audio system that the subwoofers reproduce. Suposedly the lower the frequecy the more you "feel" the bass. Thats the best quick answer I can provide, but for more info just google Hertz and read up on a few definitions.
Edit...Prower beat me, and this time I was more long-winded //content.invisioncic.com/y282845/emoticons/crap.gif.7f4dd41e3e9b23fbd170a1ee6f65cecc.gif