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Burn-in is what happens when a still image gets retained by a screen. It is very uncommon for modern LCD and LED monitors to suffer from burn-in, but it can happen if an image is left in the same position on a screen for multiple days at a time. Plasma TVs are very prone to burn-in, and OLED displays can suffer from temporary burn-in if an image is left on screen for many hours at a time.
The best way to clear burn-in is to play a video designed to correct burn-in on Youtube. These often contain rapid flashing colors and will need to run for hours, so please do be aware of this if you are sensitive to such things. If all else fails, you may need to contact the manufacturer to repair this issue.