Ok, so let's talk about variable refresh rates, or as it is more commonly known as G-Sync or Free Sync if you are an AMD fan.
First, we need to figure out the differences between the refresh rate of the screen and frame rate of the monitor. The monitor's refresh rate is ALWAYS the same, so if a monitor is 60hz, that means it refreshes the panel 60 times a second and the same goes for 120 or 144hz screens. The video card outputs different framerates based on the complexity of the content, the bandwidth, the resolution and the clock speed of the GPU. So, for example, if the graphics card is outputting 24 frames on a 60 hz monitor, you will get stutter because the frames do not catch up to the refresh rate of the monitor and the image stays on the screen longer. If the video card is outputting more frames than the refresh, you get tearing because the display is trying to show more frames than it was designed to. either way, the refresh rate and frame rate are out of sync.
What variable refresh rate does is it adds a piece of hardware to the monitor to allow the monitor's refresh rate to be variable at the same rate as the video card is outputting its frame rate. this chip in the monitor allows the video card and the monitor to talk to each other and report refresh rates to one another. So, with variable refresh rate, if the video card is about to output 139 frames a second, the monitor is getting ready to set its refresh rate to 139 hertz so that at no point during the game do you get stuttering or tearing. G-Sync is Nvidia's technology which entered the market first and Free Sync is AMD's answer to Nvidia. Both of these technologies only work on Display Port input, and certain settings become disabled for monitors, but it is so worth it.