Means, Parents on shoulder. It is used to tell the person you are talking to that your mom or dad is around and/or looking, so they don't type anything that your parents wouldn't want to see.
iSaBeLla<3: OMG i gave cody a bj last nite!!!
iSaBeLla<3: wad was the answer to question 4 in the textbook for science?
S.A.R.A.H: ok my mom is gone
S.A.R.A.H: OMG realli how was it?!?!, was his dick big?
iSaBeLla<3: it was fucking awesome! it was the biggest dick i ever seen!!!