Basically, Lemmy communities and Kbin magazines are federated as groups to the micro and macroblogging fediverse. People in friendica, mastodon or Firefish can interact with Lemmy and kbin by mentioning a community/magazine in their posts and following them the way they do with other types of federated groups (like guppe, chirp, friendica forums, etc)

Pixelfed didn't have federates group support, meaning that its federation with the threadiverse was bad, not to say practically non-existent. Supporting federated groups and even having its own type of group will allow people on pixelfed to interact with Lemmy and Kbin the way other fediverse software do: following them and tagging the community/magazine handle in posts.

short summary: It means better federation between Pixelfed, and Lemmy and Kbin.