A Real Person is someone who exists in the Real World. The usual ones the PPC comes into contact with are actors, occasionally authors, and sometimes musicians. If you really do need this explained further, you might want to stop thinking about the Fourth Wall, or you could end up taking a short holiday in FicPsych.
Usually the reason the PPC comes into contact with Real People is that an enterprising badficcer has written Real Person Fic, usually involving the actors they find "hott" from the Movies of a fandom. The fact that the Pit of Voles does not allow Real Person Fic does not put them off.