You'll probably get a more complete response from BDSM_Tourguide, but what I think of when I see that term is a bra fashioned from rope, thin chains or leather straps whose purpose is to 'lift and separate' but not to conceal anything. They're best if they're constrictive, to make the nipples stand out and maybe cause the breasts to bulge. Obviously, there are safety concerns....

Bondage bras are made by professional companies (like Spartan/Centurion), but most I see in photos are homemade.