Phrase describing if a girl is sexually active or not, often in reference to teenaged girls. The use of "pole" comes from the likeness of a male penis to a pole, hence, if a girl is on a pole, she'd be sexually active.
"I think it's pretty clear at this point that Sarah Palin's daughter is on the pole."
"You'd better stop letting your daughter dress like Britney Spears or she'll be on the pole before you can say the words R. Kelly."