Adaptive sync is a technology that allows a device to communicate with a monitor and synchronize the monitor refresh rate with the framerate of video produced by the device.  A monitor that does not have adaptive sync may appear to stutter or lag if the game’s framerate is not locked. This happens because a PC may send a frame to the monitor when the monitor is halfway through a refresh. The monitor may show half an old frame and half of a new frame. In some cases, frames may hang, which causes stutter. Adaptive sync solves this problem. The PC will only send a frame in sync with the refresh rate, so the monitor will never refresh with only a partial frame.  AMD FreeSync and Nvidia G-Sync are versions of adaptive sync designed to work with video cards from each respective company. It’s best to buy a monitor that has official support for the adaptive sync standard used by your video card.