Bra
A cream-coloured bra on a bed
A bra or brassiere is an undergarment (underwear?) for breasts. A bra is for comfort so your breasts don't fling and turn in every direction when you move, but I suppose today it's used mostly for looks.
You don't have to wear a bra if you don't want to.
I keep seeing statistics saying a large percentage of women (and presumably adolescent girls) are wearing the wrong size bra. Maybe y'all should see a bra fitting specialist (not Victoria's Secret).
Wearing a bra won't stop your breasts from hanging lower as you get older. It looks like breastfeeding? doesn't affect your breasts hanging lower, either.
Some women are going without bras because, well, they're uncomfortable and who cares what men think? I can't argue with that, as long as they're still wearing shirts.
Snapping or otherwise touching someone else's bra straps is sexual? behavior. So definitely don't do it unless you have enthusiastic consent.