A cute girl
whose sole redeeming quality
is that they look good on your arm, next to you
A nice symbiotic relationship - they get to be seen with somebody as cool as you, and you get to decorate your arm for a while. No actual emotions are usually involved. Hopefully not, anyway.
If the arm jewelry in question turns out to have more to him (her) than a pretty face, then they transcend the moniker to become a real person.
Compare with fish; no sex is implied by the term "arm jewelry".