A collar goes around a neck. For a human, it’s the neckband of a shirt or jacket. For an animal, it’s a neckband that attaches to a leash, kills fleas, or makes a poodle feel fancy.
The color of your collar says something about what you do. “Blue collar” refers to people who do manual labor while “white collar” refers to people with desk jobs. Animals generally don’t care what their collars look like. When it's a verb, collar means "apprehend" or "arrest," as when a police detective finally collars an elusive bank robber. This meaning arose from the 17th century use of collar, "grab someone by the neck."