Burn in is a common occurrence in CRT monitors. When a static image (that is, non changing) is displayed for a long period of time, the pixels are actually burnt, leaving a shadow of the image etched into the glass.
A common example of a static image on your computer would be the start menu button; even though the images on the screen change all of the time, the start menu button is always present. On an old display, it is very common to see the start menu button burnt into the glass.
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