system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Is Vsync simply a lock to FPS in relation to monitor refresh rate or does it do anything else beside this?
gpt
Imagine you have two people copying a series of pictures. One guy is drawing the picture, and then projecting it onto a wall a few yards away. The second guy is then sitting there and copying down what is displayed on the projector.

Generally speaking, it would work fine (Assuming the second guy is good at copying). You'd end-up with two stacks of pictures that are identical - One being the original copy, and the other being the copy made by the second guy.

More then likely though, you'll quickly notice that they're *not* identical. The problem is that the first guy wasn't drawing pictures and then displaying them at the same rate the second guy was copying them. So, for example, the second guy may be half-way through a copy, and then the first guy is done with the next picture and switch it out on the projector. The second guy just keeps copying though (He doesn't even noticed the picture was switched!), so the second-half of his copy is now from the second picture. (The first half was from the first displayed picture, the second-half is from the second displayed picture).

Obviously this is an issue. The easy solution is to simply have the second guy signal to the first guy when he's ready for the next picture. So, for example, when he finishes creating a copy of the first image, he holds up a flag that the first guy can see. When that happens, the first guy switches out the picture on the projector, and then starts on the next image. Then the second guy copies that next picture, and then puts the flag up again to signal for another picture. By doing this, as long as the first guy can draw pictures that fast, the two sets of pictures will be identical.

This is generally how your monitor works with your graphics card. Your graphics card works and draws the picture, and then displays it to the monitor (Like placing it on the projector). While it displays it to the monitor, it starts drawing the next picture at the same time. The 'Vsync' signal is the flag, and it's the monitor telling the graphics card that it's done copying and displaying the current picture and to give it the next one. If you have a 60hz monitor, then it displays and asks for a new picture 60 times a second.

As you noted, the Vsync signal allows the program to display high FPS content without the problem of the graphics card and the monitor getting out of sync (Resulting in the monitor only displaying part of the picture before the graphics card switches to the next).