What is "retcon"? I heard it used by a private security actor a few years back, but I don't understand what it is.
It's a contraction of "retroactive continuity". It's when an author changes some details about a fictional character and claims that not only is Megaman no longer the long-lost heir to the Kingdom of Arglegarble, he never was, and all the old stories that mentions that relationship should be ignored (on that point).
In his first book, Peter O'Donnell's heroine Modesty Blaise was said to be a refugee child in the refugee camps shortly after WWII. However, each new book took place a few months after the previous one but was published years apart, so eventually Modesty Blaise's perambulations in refugee camps was changed to shortly after the Suez Crisis (IIRC). Turns out she wasn't even born back in WWII days, and what's more, she never was.
Comic books are a fertile field for that sort of thing. Authors (or editors) tend to reboot comic book heroes every seven years or so,
Or, to grab an example
completely out of the blue

, by the Book 2 rules the
Kinunir could be built at TL 10[*]. Later it was established that you needed TL13 to build jump-4 drives (at least in the OTU), so the TL of the
Kinunir was changed[*]. The
Kinunir was not a TL10 design, and what's more, it never was.
[*] Or so I've been told; I haven't actually checked for myself.
[**] Sadly, GDW muffed it and change it to TL15, but let's not go into that. Painful subject. 
Basically it's the intellectual property owner telling you "Forget what I told you before; this is what I'm telling you now, and it's what I say goes."
Hans