VSync basically just caps FPS to a certain point, not letting it go above that point. Why would you want this? Each monitor has a refresh rate. Most monitors are 60Hz, while the higher end ones can be around 120hz. The refresh rate basically tells you the maximum amount of FPS your monitor can handle before doing this, known as tearing:
I only have a 60hz monitor and the tearing in openBVE is insane... so I have to enable VSync to cap it at 60 so the tearing won't be everywhere. For those who don't get 60, you can leave it disabled but for those who get openBVE to run above their refresh rate, VSync should prevent any tearing.