It's short for vertical sync, where the flipping of a double-buffered fullscreen window (or regular window) is synchronized with the refresh rate of the monitor. This way it only flips the back buffer to the front at the start of the monitor redraw. If you don't do this, you get screen tear, where the top half of the monitor draws the old front buffer, and the bottom half displays the contents of the new front buffer (which is usually 1 frame ahead so animations look off).
But is that really really noticeable ? I've been working with LWJGL for a few months now and I have Vsync disabled since then , and I could never notice this effect you described .