Communities in Lemmy/Mbin are not federated by default. So when you create a new community, it will only be available to your instance. At least 1 person from all other instances must follow it in order to make it available. This tool does that. It follows your community from all remote instances until at least 1 other person follows it.
Some other thread had me trying to remember this the other day, so cheers on posting it. Might be worth adding to a pinned thread/sidebar as a resource (alongside various newcommunities and promo communities).
Good point, I’ll add it to the sidebar later