A bandeau bra is shaped like a strip over your breasts, sometimes with ruching in the middle. They are kind of a cross between a strapless bra and a tube top. They are made of stretchy fabric so don’t normally have a fastening in the back, instead they go on over your head. Like many strapless bras they often come with detachable straps so you can wear them with or without.
As there is little to no shaping this isn’t a supportive bra, it really only works well for smaller breasts. If you are bigger and need something more supportive you are better to go with a classic style strapless bra. You can get variations of bandeau bras that have wires and padding to give more lift but a classic bandeau bra is really just a strip of stretchy fabric. In saying that they can be a fun piece of stretchy fabric that often come in exciting colours and prints, and can be really comfy for smaller busts.
The bandeau shape has been used in tops and bikinis for a long time, so unlike most bras they don’t have to strictly be underwear, often they are worn to show under clothes for a layered effect.