Well, it generally refers to a process that allows you to accommodate a certain sized toy or fist. You begin with a smaller toy, and depending on the end goal, you either take multiple sessions to get there (like, if your goal is to take a fist), or you take an extended time within one session (like if you're goal is to be pegged with a small/normal sized toy).  This is from a dominant/top viewpoint. If the lady is submissive, she might be open to butt plugs, larger anal toys, etc.  

It's a ton of fun from the dominant/top side of things! I don't offer that as a switch/submissive, but I'm sure the ladies that do offer it submissively enjoy it. If you're interested in anal training as a submissive, I'd suggest seeing someone who is experienced and enthusiastic about it.

Not even kidding, that's what they're called!
And the last one...is not happening. Lol

They aren't for sharing though, so I'd listen to Miss Erin on this one