It's the rare movie, book, or…anything really when we can refer to a pair of teenage girl's underpants as a prominent symbol. But here we are.
We're not going to describe them in detail, because that's insanely creepy. So what's the point of the underpants besides being an attractive lure for pervy teenage boys?
Well, the Geek's supposed to claim this trophy as proof for his friends that he has had sex with Sam. Women as trophies is a recurring motif in this film. Sam gets the official status of "cool girl" for giving the Geek her panties even though they don't have sex—her panties are a little bargaining chip. A thank you to the Geek for listening to her problems and helping set her up with Jake.
Jake returns Sam's panties to her at the end, which helps him win her heart.
SAM: Thanks for getting my undies back.
When Jake tells her to make a birthday wish, she said it already came true. We imagine the wish was for Jake, but maybe it was just to have her underpants returned to her. Who says no one wants socks and undies for their birthday?