That is a big, fancy word, I'll tell you what. Even breaking it down doesn't help a whole lot. Meta means something like "a level beyond" and the second part is theatrical from "theater." In essence, you have something that transcends the theater.
Of course, that definition doesn't help a whole lot because it is horribly hard to understand, so let's put it in other terms. Meta-theatricality, as commonly understood, is when the people in a play acknowledge the fact that they are in a play. They usually do this by either talking to the audience directly, or making some other side comment about the fact that they are acting. Some plays do that, where an actor hold his hand up to his mouth and gives an "aside" to the audience to let them know something important about the plot or one of the characters (consequently, though it is not a play, the TV show "Malcolm in the Middle" did this quite a bit, with the main character addressing the viewing audience. "The Bernie Mac Show is another example, with him sitting in his chair addressing "America.")
There are other, more complicated and less accepted variations on the definition of the word, but this is the most commonly thought of one.