A bit gag is a cylindrical tube which is held in the mouth. It is usually made of leather or rubber and fastens around the head leaving the wearer unable to speak. It is a common form of bondage gear.
More About Bit Gag
Bit gags can be fun for those interested in stepping into the world of BSDM, but wearing one will leave the wearer unable to speak. Couples may want to you come up with a safe sign, something similar to a safeword, to use if the wearer feels uncomfortable and wants to stop.