A burn in may refer to any of the following:
1. A burn in describes the process of turning on and leaving on an electronic device for a predetermined period of time to help prevent any premature failures. Often a computer or other electronics maker will perform a burn in before shipping a device to help identify and prevent failures.
2. Alternatively referred to as a ghost image, burn in is an issue that can occur when a non-moving image is displayed on a computer monitor for an extended period of time. The image can burn into the phosphors of the monitor, remaining faintly visible even when the monitor is off.
This issue occurs most often with older CGA and EGA CRT monitors, but can still happen with new CRT monitors, although not as common. To help prevent a burn in, turn the monitor off or use a screen saver when not at the monitor.
If this happens on your monitor, try degaussing the monitor to fix the burn in image. Unfortunately, most burn-in's are permanent and never entirely disappear from the monitor.