A gag is used to remove a slave's ability to speak. Typically some sort of object is placed in the mouth and is fastened behind the head so that it cannot be easily removed.
Gags come in many different shapes and sizes. Ball gags, bit gags (like a horse's bit), penis shaped gags or pieces of cloth are all examples of common shapes. Gags are very common in BDSM because they remove the ability for the slave to complain or speak their own opinion. By losing this control, a slave is reduced to only making muffled noises or moans.