Definition - What does Drop Collar mean?
A drop collar is a type of collar in V-shape, which is otherwise similar to your usual BDSM collar. The V can appear in two ways: either the straps of the collar meet in a V shape, or a V shape is cut from a wider piece of material. There is usually a ring at the bottom of the V for attaching a leash or chain.
Kinkly explains Drop Collar
A drop collar is simply another style of submissive/slave collar that's considered generally more comfortable than a straight one because they follow the natural shape of the neck and collarbone. Pulling on a leash from a drop collar can also be less strenuous on the neck than a regular collar.