It basically means that we are seeing content that has been posted or created across the "Fedeverse" for example Lemmy, if i understand correctly (Extremely summarized, there's more to the federation).

Earlier, we only saw content/posts made locally on Kbin; however, it appears that we are federated, which is providing us content from other instances in the Fedeverse. This very post is from Lemmy.world, but i'm replying to you from Kbin.social, if that makes sense.

Firefox on my pc and Safari for Kbin on my mobile.