I know very little about this kind of thing, I'll try though!

Sometimes a technology called Vsync will synchronize your monitor's "Hz" with the game's FPS. So the game would default to cap at 60 FPS on a 60Hz monitor. I think it's only available with Intel 2nd/3rd gen CPUs and select graphics cards. Though it doesn't affect your gaming experience much at all. A 60 Hz monitor is perfectly fine, I see no need for 120 Hz.

Instead of me summing up what is to be said about 60hz and 120hz, please refer to this article. It explains everything about hz in great detail, and saves me time NOT explaining and typing it all out.

[ Register or Signin to view external links. ]

p.s. next time you could just google the subject first instead.