refers generally to the synchronization of frame changes with the vertical blanking interval. Since CRTs were nearly the only common video display technology prior to the widespread adoption of LCDs, the frame buffers in computer graphics hardware are designed to match the CRT characteristic of drawing images from the top down a line at a time by replacing the data of the previous frame in the buffer with that of the next frame in a similar fashion. If the frame buffer is updated with a new image while the image is being transmitted to the display, the frame buffer gives it the current mishmash of both frames, producing a page tearing artifact partway down the image.
Vertical synchronization eliminates this by timing frame buffer fills to coincide with the vertical blanking interval, thus ensuring that only whole frames are seen on-screen.
Computer games often allow vertical synchronization as an option, because it delays the image update until the vertical blanking interval. This can cause lowered frame rates due to latency (the period of the refresh rate at maximum), which might be undesirable in games that require fast response (e.g. first person shooters).
VSYNC is also the name of the signal indicating this frame change in analogue RGB component video.