An afterimage occurs when visually perceiving an image after you are not looking at the stimulus anymore. For example, if you look at the sun for a bit and then look away you can see the afterimage of the sun even though you are not looking at it anymore. A negative afterimage sensation of opposing colors that occurs after staring at a colored stimulus.
For example, if you were to stare at a green sheet of paper for an extended period of time, and then switch your focus to a white sheet of paper, an afterimage would appear on the white page. Negative afterimages are cause by stimulation fatigue from your eyes' receptors. Positive afterimages occur less frequently, last a short duration, and are the same color as the original stimulus. For a positive afterimage to occur the stimulus must be very bright (like the sun).