From this recent blog post, Recently PieFed added a way to group communities into collections of related communities, which we called a “Feed”. Unlike Topics, Feeds are federated, can be created by anyone and can be public or private. There are now hundreds of feeds at https://piefed.social/feeds.

This lets niche topic communities continue to spawn across instances (thus avoiding all-eggs-in-one-basket issues) but makes it more convenient for users to see all similar content in a single place.