A coffin, in the context of BDSM, is not used for burial but rather used as bondage equipment designed for extreme confinement. In most dungeons and clubs, coffins are made in the traditional hexagonal shape that is common in cinema (and specifically in vampire movies). Coffins may be lying down or standing upright. Aside from the large, lockable lid, many BSDM coffins also have smaller hatches to give access to the breasts or genitals without releasing the sub inside.
Coffins are padded and put very little stress on the body, so they’re suitable for longer scenes. It’s important to ensure proper ventilation, though.