For anyone familiar with forums it makes sense to describe it like connected forums.

You sign up for a forum, and most of them have decided to share posts with each other. Because of the vast number of forums someone on your forums needs to look at other forums and add them to your forum so that they show up so that the traffic is limited to what the people on your forum are interested in. Like discovering new forums, but without needing to go to each forum separately.

The reason not all of the forums talk to each other is that the people who run the forum don’t want spammers and assholes cluttering up the place, so they don’t let people link to those forums.

Plus you can always sign up separately for the asshole forums or create your own forums if you want to. If you create your own, you have to maintain it just like any other forum.

As Ling as they don’t focus too much on forum structure it at least covers the connection part with a familiar context.