Definition - What does Collar mean?
A collar is a restraint that is sometimes placed around the neck of submissive partners in BDSM play. It is not uncommon for dominants to wear them as well. Often, submissive partners that regularly wear collars are referred to as being "collared." At times, formal collaring ceremonies are held. These ceremonies can be likened to wedding ceremonies, with the collar being nearly equivalent to a wedding ring, as they signify the start of a special relationship between a Dominant and a submissive.
Kinkly explains Collar
Collars can be made of nearly any material, including rubber and even metal, but the most common type of BDSM collar is made from black leather. Many collars also have one or more metal D-rings or O-rings making it easy to attach a leash. Collars might also be plain or adorned. Many professionals who enjoy BDSM forgo the usual collar and opt for other methods of collaring and use a necklace instead.