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.