VSync = vertical sync of your monitor. It means that the page flipping is synchronized with the display. It reduces screen tearing artifacts but lowers the refresh rate of the game to the one of the monitor. Whether this is bad or not is debatable. I can't stand screen tearing so I leave it on all the time. 240fps is worthless if the monitor can't keep up.
and it's more than any monitor can handle! You only wasting energy by rendering the image 3 or 4 times during one display cycle. The VSync-capped 85 fps I get are more than enough.