Screen tearing
Tearing is a horizontal split across the screen where the top and bottom show different frames. It happens when a new frame arrives while the monitor is part way through drawing the previous one.
In practice
Tearing is most visible when the camera pans quickly in a game or when you scroll fast. It happens most when the frame rate is higher than the refresh rate or not synchronized with it. It is a timing issue rather than a hardware fault and it can be fixed entirely with the right settings.
How to check
Run the refresh rate test to confirm the refresh rate you are really getting. Then turn on variable refresh if your monitor supports it or V-Sync if it doesn’t. Capping the frame rate just below the refresh rate also helps. If the line appears on the desktop rather than in games, check the graphics driver settings.