I was one for two years
interesting experience.
I will say that it means different things to different people. Some people will say it is a lifelong commitment like marriage. Some people will say it means "no limits" (id est, you can "never say no"). Some people will say it is a loveless relationship where the dominant figure must necessarily be sadistic and cruel. And there are an equal number of people who say opposite things and things in-between.
The BDSM community can't even come to a consensus on what the difference between a submissive and a slave is, and it sure can't come up with a difference between a slave and a collared slave (except that typically, a collared slave has some form of literal or symbolic collar).
In the end, despite what fun/not fun stories you hear, it is what you make it, literally. Being a collared slave is what you decide to do who you you decide to do it with. There are no "universal" rules or universal experiences, and anyone who tells you otherwise is full of shit.
Also, fetlife.com is an interesting place to do research, if you don't mind dick pics. Specifically, check out this group:
https://fetlife.com/groups/219