It is given to the player when entering the Villa of the Sierra Madre. It is also used to get Dog and God to listen to you when you find him, and to turn God to Dog.
Are you listening? From now on, when I talk to you, pay attention. I've left markers on your pip-boy. Find the three other collars and tell 8, 12 and 14 get them to the fountain. Obey me and you can all go free.