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.
Yeah I was honestly surprised this got posted and nobody said something like “the entire point of not automatically federating in new communities is to cut down on resource consumption,” because to be honest? That was my first reaction. If nobody on a server is subbed to a certain community, it will not pull in its content, which is probably very helpful for smaller home-run instances. This kind of defeats that purpose. Although its damage is limited by the fact that only people actively engaged in creating a community will try to use it, and it will probably only be used by new communities.