Plunge bras are designed so you can wear low cut clothes without your bra showing. The cups join together low between the breasts, with a thin centre piece. Usually the shoulder straps are wide, so you can wear any necklines that are widely cut as well as low.
Plunge bras can also be push-up bras, this will give you fuller cleavage than a plunge bra with wider set cups.
When you are buying a plunge bra try leaning forward in it while you are in the changing room. This is to check your breasts don’t fall out the front, which can happen if it isn’t the right fit for you.