It's when they take elements of the old movies, usually mistakes or inconsistencies, and try to weave them into canon. For example, they puposely showed Jango Fett bumping his head in AOTC as a way of explaining the stormtrooper who bumps his head in ANH - according to Lucasfilm, Jango's clumsiness was passed on to his clones, which explains the stormtrooper bumping his head in ANH. Totally lame. I think it's short for 'retrospective continuity', as in retrospectively trying to fit something into the continuity even though it wasn't originally part of it, but that's just a guess.

If I'm wrong here, feel free to correct me.