V-sync is a term used to describe how a graphics board renders and displays an image to the display. With v-sync On, the graphics board draws one frame of a scene, and waits for the monitor to display it before starting the next frame.
With v-sync Off, the board draws one frame, and then immediately starts on the next frame - whether the display is ready for it or not.
With v-sync turned off you may experience tearing which can be quite disturbing. However if you hav bad frame rates you can earn some frames by turning v-sync off.