Retcon, or ret-con, is short for "retroactive continuity." In essence, it's taking an known intellectual property, changing something important about the backstory, then declaring "that's how it's always been."
As an example, let's pretend a new writer is redoing Superman and says that he was a mutant from Atlantis instead of an alien from Krypton. Or that Spiderman's Uncle Ben was killed by a cop instead of a common thief.
Depending on the circumstances, be it a "What If" comic from Marvel or an Elseworlds graphic novel from DC Comics, retcons can be interesting speculation. The problem comes when the main storyline that people love is changed, for reasons that are vague and/or arrogant, like in the many cases with comic writer John Byrne, who has always had a general attitude of "I know what makes better stories more than the creators of the property I'm working with."
#Where this applies to League of Legends
In September 2014, a giant retcon was introduced to League of Legends, basically removing the Institute of War from the game and limiting the role of Summoners (that's us) in the game world itself. The team in charge of the rework claims that when the work is done, the story changes will allow for better stories released more frequently.
However, this drastic change has caused multiple problems, not the least of which is that we're currently dealing with an incomplete mess of a story, a puzzle with missing pieces as well as pieces from previous puzzles mixed in. Then, there's the tech issues about new content not being able to be released onto the website because... reasons. Also, Narrative does not seem to want to talk to fans at all, going back into their ivory towers and giving us their arrogant and contemptuous silence.
In other words, Narrative has a lot of work to do to fix but they're under no pressure to do that work as quickly as possible. They're more interested in tweeting about their D&D and WH40K games than producing content and fixing their long-standing communication issues.
I hope this helps.