This is teenage Instant Messaging slang meaning "Parent Over Shoulder". It tells the other user the their mother/father is standing over them, and to not say anything that might get them into trouble.
Girl 1: LYKE OMG TYLE IS LYKE DA HOTEST GUY IN DA HOLE WORLD!!!!1!1
Girl 2: P.O.S.
Girl 1: can u help me wid my english hw?
Girl 2: sure, wat pg is on?
Girl 2: KK, hes gone
Girl 1: I can tell tyler wants meh, he wus starin at meh b00bs in eenglish class...