The word is a shortened form of retroactive continuity and in modern times is most often seen when talking about comic media and science fiction/fantasy, with daytime soaps and primetime sitcoms in there for good measure.
The qualities of these retcons vary greatly, often depending on how closely a particular franchise follows any sort of internal continuity, or canon. The most famous example of canon is the various Star Trek movies, TV series, and related media. As such, it contains notable examples of retcons, from the look of the Klingons, to the importance of currency in the universe, to an entire period of history known as The Eugenics Wars. Some of these are explained away during subsequent episodes of more recent series in the franchise (with varying degrees of success), while some are simply ignored.
Another franchise that deals in frequent retcons is Doctor Who. Over the nearly 60 years of the series, it has struggled to sustain a consistent internal continuity. During the Classic series (1963 - 1989) there were many times when the series contradicted itself, and rarely did the writers do anything to explain these continuity errors. The New series (2005 - present) has been more successful in maintaining canon, although it, too, has had issues. But, what they have often done is made attempts to retcon some of the continuity errors from the Classic series when they've affected events in the New series. As the Doctor might say, "wibbly-wobbly, timey-wimey".
Of course, some of the most hilarious examples of retconning come from the sitcom "Friends", where the backstories of nearly every main character changes from how they were introduced at the beginning of the series. While some were more major retcons than others, Phoebe's backstory was in a constant state of flux, to the point where I don't believe even the character really knew what their true history was.
On the deck is the first retcon of Furble's run. But first...
|Strip 57/166 - Apparently he'll eat anything!
Our intro to the character Bitem continues. He's a hungry little snake, which makes sense since it's the middle of winter and most of his prey is probably busy being scarce. But where does such a little snake put so much food? Your guess is as good as mine.
|Strip 58/166 - And I mean ANYTHING!
The way in which Bitem holds his head up as he interacts with Furble and FC was actually inspired by Slimey, Oscar the Grouch's worm friend on Sesame Street. He's probably one of my favorite classic Muppet characters, just a simple worm controlled by two control rods. But, like all of Henson's creations, still able to express a wide array of emotions thanks to expert puppetry (or Muppetry).
|Strip 59/166 - Why? What did Furble do to you?
The continuity error: our mystery Admiral, last seen scurrying away from Furbulia after he failed miserably in his attack on the planet, has made a triumphant return, and he's here for Furble! Sounds ominous, yes? One problem: he doesn't know Furble. Didn't meet him, didn't even make it to the planet's surface. So, how do we resolve this mistake?
|Strip 60/166 - You know that feeling when someone is behind you...
The retcon: Furble and FC "remember" the Admiral as he escapes his decimated ship, thus completely ignoring the fact that they never met, didn't even know that he had plans to attack Furbulia. Again, I don't know precisely what I was thinking here, other than I wanted the Admiral to be a "big bad" for the comic strip (for a bit, at least). Perhaps I knew I would want a chance to talk about retconning sometime in the future in a blog when I revisited these comics. Yeah, that must be it.
There, I just retconned my life.