V-sync makes your video card to sync up with your monitors refresh rate. The monitor refreshes 60 times per second on most monitors. If your frame is not finished being rendered when the refresh starts, the video card has to wait until the monitor is finished updating the screen.
It is used to prevent having two partial frames from being displayed at once, resulting in tearing. You'll find most people prefer to keep v-sync off unless you can achieve higher FPS than your refresh rate. Nvidia's Adaptive V-sync does this automatically, but mostly, people have to choose to use it on high FPS games, and turn it off otherwise. I also find with a 120hz monitor, that I can just leave it on, as missing a refresh period isn't has big of a hindrance, when the monitor refreshes twice as fast.