Dodo
said
:
something that gets rid of screen tearing in games but kills your fps
^, basically.
It caps your FPS to whatever Hz your monitor is, probably 60 if you have a standard monitor.
Say your game is running at 100 FPS, and your monitor is 60 Hz (aka the most it can display is 60 frames per second), you will be getting 60% of the 100 frames your game is running at.
Tearing occurs because your monitor is only showing some of the frames (and sometimes incomplete frames), due to 40% of the FPS being 'wasted' since your monitor cannot display that many frames each second.