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"RETCON" is a shortening of the phrase "retroactive continuity".
It refers to a story or statement which in any way alters previously-established history
in an ongoing series.
A "retcon" can be as innocuous as Roy Thomas' All-Star Squadron (which coined the phrase) or Kurt Busiek's Untold Tales of Spider-Man. These series simply told stories set in a particular past time, which painstakingly didn't contradict any of the previous stories. For example, there was never an explanation in the 1960's of why Peter Parker stopped wearing glasses; an Untold Tales of Spider-Man story explained how it happened.
At the other extreme, a "retcon" can be a complete dismissal of all past history. Examples of this are John Byrne's Man of Steel mini-series in 1986, or the Legion of Super-Heroes stories published after Zero Hour in August 1994. Man of Steel retold the origin of Superman, implicitly declaring that all previously-published stories about the character never happened, and that previously-established facts were no longer necessarily true. For example, in the 1930's it was established that Ma and Pa Kent died when Clark was a young adult; Man of Steel stated that they were still alive.
In between these two extremes, there are stories which merely revise past history, either putting a different twist or revelation on it (e.g. Barry Allen travelling back in time and acting as the lightning bolt that gave him the powers of The Flash), or changing details that are no longer desirable (e.g. updating the Fantastic Four's accident in space such that it took place in the recent past rather than in 1960).