Screen tearing is a graphical distortion that occurs when the graphics processor is out of sync with the screen. This causes a horizontal line to appear when playing a video or video game because the top region is out of sync with the bottom region.
If the GPU is heavily loaded, it may not be able to keep up with the refresh rate of the screen. As a result, part of the screen is redrawn in one frame and the rest in another. When this happens in several images, a noticeable vertical line appears on the screen.