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. Some agents technically are real people in that they stumbled through from the Real World to the PPC. Mary Sues are definitely not real people by any stretch of the definition.