A stuffed leather gag is a device very much like a ball gag or penis gag that is used by a dominant or top to silence a submissive or bottom during BDSM activities. They are very similar to ball gags, so you may look at that node if you like. Thanks to human anatomy, stuffed leather gags will do about the same level of silencing job as a plain old ball gag. They are easier to wear and less attractive than a standard ball gag, if you have to like the view of a bigass ball stuck in someone's mouth.
The major differing factor between a stuffed leather gag and a ball gag is that the stuffed gag is made of leather instead of plastic, rubber, etc., and is stuffed with some manner of pliable material, perhaps foam or more leather. This gives the plug a non-ellipsoid shape, meaning that it is often rather flat with a bulging middle, rather like a very fat tongue.
Many people find these gags to be more comfortable than a ball gag, owing perhaps to the different geometry of the insertable portion, or the softer texture of the leather. Also, some people just like to chomp on leather. As such, most gags of this sort can be worn for quite a long period of time. They will however, induce drooling, as the leather will not absorb the bottom's saliva, and it's gonna have to go somewhere if the bottom cannot swallow it.
Stuffed leather gags are reasonably priced in the gag world, and are available in many of the same configurations as ball gags, in relation to size, attachment type, and strap type. You may want to compare with ball gag, penis gag, bit gag, ring gag, and gags of all sorts.