Alas, Poor Yorick! I knew him, Horatio; a fellow of infinite jest, of most excellent fancy; he hath borne me on his back a thousand times; and now, how abhorred in my afterimage he is!
Well ... that's what William Shakespeare's Hamlet might have said, had he been looking at a vintage Pears' Soap advertisement bearing court jester Yorick's skull, rather than holding an exhumed and rotting Danish cranium. Stare long enough at the skull in the ad, and it will be “burned” into your vision even after you look away.
Afterimages such as Yorick's skull help us understand how neurons in various areas of the brain adapt to the visual environment. Adaptation, in this case, is the process by which neurons habituate to, and eventually cease responding to, an unchanging stimulus.
Once neurons have adapted, it takes a while for them to reset to their previous, responsive state: it is during this period that we see illusory afterimages. We see such images every day: after briefly looking at the sun or at a bright lightbulb or after being momentarily blinded by a camera flash, we perceive a temporary dark spot in our field of vision.