Meta means about, so metafiction is fiction that is about fiction. In other words, fiction that thinks, and even talks, about itself. It's kind of self-conscious.
So if the story you're reading is about, well, writing stories, that's metafiction. If the book you're reading acknowledges itself as a book, that's metafiction. If the author jumps into the action and says, "Look at me! I'm the author! Watch me write!" that's metafiction.
Check out our analysis of metafiction in the short stories of J.D. Salinger for more.