Retroactive continuity is a seldom used phrase for the much more commonly used retcon. It means that changes are made to the official lore of a fiction franchise retroactively, so that older lore is no longer canon. If such a change has been made, it's usually referred to as having been retconned.
Description & Reason
A retcon can either be additions made to existing lore to explain certain events, or rewriting historical facts, and are meant to have always been made this way.
The reason it is done is because when a fiction universe grows, story lines might conflict as every writer can't know every bit of lore, or there is just a small tweak needed to make the whole story line a lot more interesting.
When a retcon is made and some information has been made redundant, that information can either just be completely disregarded, or just made into flavour lore.
Example of a retcon
In the first Diablo I Manual, there was a large section detailing demons, and it was written like a text made by a Vizjerei sorcerer from the game world. It explained that the goatmen were demonic beasts bred in hell.
The goatmen were not really mentioned in Diablo II, but with Diablo III, Blizzard announced they were not at all demons, but former humans, corrupted by the Vizjerei mage clan in the Mage Clan Wars. This was explained by Abd al-Hazir, a travelling writer, who is the closest you could get to an investigative journalist in Sanctuary.
Abd also mentions the Vizjerei, and their false studies on demons, which they made just because they wanted to lessen their blame in brining about the Mage Clan Wars in the first place.
In this case, the old lore was replaced by new one, by making the old one "flavour lore".