Judder is a visual issue that occurs when the horizontal lines in video images appear to stutter or shake. This problem often arises because the frame rate of the video source does not match the frame rate of the display.

When these two frame rates are incompatible, the video cannot play smoothly, leading to a noticeable and distracting shaking effect on the screen. This can happen when, for example, a movie filmed at 24 frames per second is shown on a television that operates at 60 frames per second.

The mismatch causes the video to display unevenly, resulting in the judder effect. To fix this, one can adjust the settings on the display or use devices that can convert the frame rate to ensure a smoother viewing experience.