There's a lot of technology that's designed to make you computer's graphics look as good as possible, but it's not always clear what they do.

If a monitor and a game are having trouble syncing up, then VSync can lower your frame rate significantly to try to find a ...

On the other hand, turn V-Sync off if you have it enabled and you're noticing severe input lag or lower-than-normal framerates. However, chances are that you don't need to have V-Sync turned on.