With reference to HD TVs, when you see the term 'hz' it is specifically referring to the refresh rate of any image seen on the display. As an example, a TV with a rating of 120 Hz is able to refresh or rebuild the image seen 120 times per second.
The result of a higher Hz number is elimination (or at the very least reduction) in the blurring or ghosting effect that's sometimes visible when watching programmes showing fast action scenes, such as sports like football.
In LCD TV's it's more relevant and as a minimum you should be looking for a refresh rate of 120 Hz. Some TVs do give 240 Hz however it is difficult for the human eye to discern much difference between 120 and 240. Motion blur is more obvious normally on larger displays, and it's possible that 240Hz could give a better image on a larger screen although that may vary from person to person and depending on the quality of the content being watched.