This site lists the communities on -shall not be named- that have either migrated or are also located on different sites. It also lists whether it is the official community or a spin off. A very helpful tool for those who have rid of -shall not be named- and have forgotten communities they once followed.
I think, in this case, OP was trying to say that the site could help you find Lemmy communities based on the rules and topics of a subreddit, as the site lists Threadiverse communities (Lemmy and Kbin) and servers in messaging apps such as Discord and the Matrix network under the names of subreddits, like !196@lemmy.blahaj.zone under r/196
I meant this. Just thought it was easier vocabulary for those (including myself) who are brand new to the fediverse. In time it will change, but as of now I do still read communities as subreddits. 14 years there and only my 10th day here, but stoked to be here!