A YouFic stars you. In other words, this is a second-person perspective fanfic.
A common type of YouFic is a "Choose Your Own Adventure" story. In this, the reader may select between two or more choices. The story then directs the reader to a certain chapter or page of the book. This chapter/page will be directly affected by the choice the reader made.
Another name for YouFic is "Reader Insert." If the reader is made into a Mary Sue, see You!Sue.
When agents need to enter a YouFic, they need to use a "You" crash dummy, or one of them will become the "you" character of the fic.