A promise ring can mean two things, so I'll give you both definitions.
Def 1:
A promise ring is a token that a man/boy gives his woman/girl whom he is dating. It is a symbol saying they are in an exclusive dating relationship, and plan to take that relationship further one day. Typically a promise ring is given when a real engagement isn't practical for whatever reason (under 18, living in seperate places, no money, only been going together for a short time). She wants one as a symbol of your love and commitment to her.
Def 2:
My GF wore a promise ring, but as a promise of chastity.
Ryan