Vsync is Vertical sync. With vsync on it syncs your video card and monitor refresh rate. So your video card puts out 60 frames a second, because that is the most your monitor can display.
Turning it off shows how many frames a second your video card can really put out. One your running your video card at max full time so that can't be good, and also throwing more frames at the monitor that it can't use causes problems as well. The monitor gets confused and you'll see "screen tearing" where the frames don't line up or over lap.
Turning it off is only used for test purposes, turn it on and leave it alone until you know what your doing.