Ah, it’s described in the readme.

Catodon is a fork of Misskey that brings together the best features of Sharkey, Firefish and Iceshrimp, and adds what none of them has: Lemmy-compatible forums and federated blogs. Microblogging, forums and blogs live in one place, and it is built to make communities easier to run, easier to understand, and more comfortable to use every day.