The Online Family answers:

As you've already noticed, kids have created an online language of their own -- and even use Internet speak when they're offline. POS means "parents over shoulder." It's your daughter's way of telling her friends that she no longer has privacy. In some cases, she probably just needs her space to chat with her buddies and doesn't want them to type anything that would embarrass her. But in other cases, it might be more. As a parent, you must trust your instincts. She might be talking to someone she doesn't know well, or someone who has dangerous intentions. To play it safe, come up with an Internet usage policy for the family that establishes reasonable ground rules for whom your children can IM (such as only friends from school or people you know) and appropriate behavior (like no cyberbullying or using obscene language online).