What does it mean to be pinned by a fraternity boy?

Being "pinned" by a fraternity boy (or any college student involved in a Greek organization) means that he has formally asked her to be his girlfriend. The "pin" refers to a piece of jewelry, often a small emblem of the fraternity or a simple pendant, that he gives her as a symbol of their committed relationship. It's a more formal and public declaration of their relationship than a simple "going out" or dating.