Anastasia Luccio: Madre di Dio, Harry. I cannot for the life of me imagine what it does appear to be.
When you watch a story from the beginning, when a major plot point happens you are often primed for it because of the events of the story that build to it. Then you try to explain it to your friends... which results in blank stares, because they weren't primed the whole story for it, no matter how well you describe it to them.
This is a phrase to use when describing a complicated story point, because really, a person does not need to hear the history of Star Trek from the beginning to understand the use of a trope.
As this is a page defining fanspeak, there's no need for examples, though obviously this can happen in Real Life as well due to lack of context.