A bondage hood (also called a gimp mask) is a fetishistic hood. It may be made from rubber, PVC, or leather. It is designed to completely cover the head, and is thus accommodated with an opening at the back to extend the neck hole for adorning - which may be done up with a zip or cord. Other openings include 2 eye holes (sometimes these can be closed using zips), optional nostril openings, and a mouth opening (which is often closed with a zip).
Note that this is potentially extremely dangerous, as blocking all access to air will result in rapid asphyxiation. The safety warnings regarding gags also apply.
It is notably used in BDSM, its function being to objectify the wearer as well as providing sensory deprivation, and thus reduce them to the status of a sexual toy, rather than a sexual partner, as part of a scene. It can also serve the function of disguising their identity when desired, for example at play parties.