Piper is a waitress from the Roadhouse.
Sam and Dean park outside of a bar called the Roadhouse, a place they are familiar with. Dean encourages Sam to have fun, though Sam says he only wants to dig into lore like Castiel, in case there is a case up ahead.
The two exit the vehicle, and some time later, Dean returns and is greeted by this blonde woman, who is scarcely dressed. Sam calls from the backseat, asking if they could have some time.
Dean agrees, and gets out, but doesn't leave before he spies on the woman through the window for a minute. The woman, named "Piper" by her name tag, looks around for her golden hairpin, and asks Sam for its location.
Dean later congratulates Sam for losing his virginity again, two years after having become a born-again virgin.