In a single-medium property, it's the simple difference between what's on the screen/page/stage and what's not, what a character says versus what an actor says. We understand that line. In roleplaying, it's tagged as in-character or out-of-character (OOC).
It's the difference between the rules that structure a game and the story of that game.
It's the line between what's in-universe, in-story, in-game, and what's not, what's clearly the construct supporting the story. And in transmedia, there's a danger that these lines become blurred. My instinct, though, is that this should never, ever, happen.
In a transmedia property, you might have a website for your driving platform -- a movie website, say, with trailers and behind-the-scenes extras and release dates. But then you might also have a corporate website for a fictional company in your storyworld. How do these two websites interact?
The movie website is clearly "out of character." It's aware that the property takes the form of a film, that the story is fictional. I call this meta-content, content that is self-aware it is content.
The corporate website is "in character." To all appearances, it should look like a real corporate website of a real corporation. This allows it to be fully immersive, or "pervasive." This website does not understand it is fictional; it believes it is real. This is in-universe content.
There probably needs to be a road of some kind between these two kinds of content, a method of discovery.
Perhaps you find a rabbithole by watching the film and seeing the website advertised, and you go there to check it out. At that point, you know it's part of the fiction of the universe, and can immerse yourself in it.
But the movie meta-content website can also act as a gateway of sorts. It can host links to in-universe content. The audience understands that the main website is out-of-character, and so it isn't jarring to see side-by-side links to opposing organizations. It's a way to draw the audience into the extra digital content.
But that road needs to be only one-way. The corporate website should not have a link back to the movie website.
As soon as it does that, it acknowledges that it is fictional, that it's part of a story universe, and that immediately pulls the audience out of their immersion. It's a blatant breaking of the fourth wall. It's like if, in the middle of the Avatar, Grace turned to the camera and said, "Man, these special effects are pretty nifty, eh? I hear the video game's going to be just as good. Check it out."
Meta-content can be a gateway to in-universe content, but the reverse cannot hold true.
The same follows in other situations. Perhaps you want to post video summaries of what's happened so far in the narrative. Great. But those videos should not be narrated by the main character. He doesn't know he's in a story that needs summarizing to the audience, so he cannot do it.
(Unless, of course, he is aware: perhaps he's the leader of an organization that participating audience are "members" of, and he's summarizing what's happened so far in-character to them. But it must remain in-character.)
All content for a transmedia property can be described in terms of meta-content or in-universe content, and that divide between them needs to be crystal clear. There should be no blurry lines -- in the understanding of the creators or the audience. They need to understand what they're looking at in order to understand how they're meant to interface with it.