Vertical sync, or vertical synchronization, refers to a setting on a PC graphics card that limits the card from outputting more frames per second than the display monitor's refresh rate will allow. This is done in order to reduce distorting effects caused by a frame-rate that is too high.
The maximum number of frames that can be shown per second on a display is determined by its refresh rate. A refresh rate of 60 hertz allows for 60 frames to be shown per second. If vertical synchronization is enabled, the graphics card will restrict output to the monitor to a maximum of 60 frames per second, even if it is capable of rendering more.