they wanted something to tye all the cullen kids together in the film so the boys wear the arm bands and girls necklaces and broch. It is mentioned a little in the book
i's in the movie. if u look at the boys arm u'll c a wristband that is black and the girls wear different necklaces around their neck like alice's collar is called the choker and rosalie wears the pendant. the symbol on it means- the hand=faith,sincerity the lion=strength and ferocity and the trefoil=perpetvity. both collar and wristband have those symbols on it