Fediverse is Stand for Federated Universe,it's like the decentralized social media(mastodon,lemmy, pixelfed,etc) they are use activitypub protocol to connect each other so you can interact with mastodon(or other fediverse social media) user with your lemmy account,in fediverse you can create your own server(instance),While reddit, Twitter, Instagram is centralized social media Controlled by big tech/corporation.
We "federate", meaning we don't give a shit who owns stuff, so we use "activitypub" to let Lemmy users access posts from other services like PixelFed (new nerd Instagram) and Mastodon (new nerd Twitter) without any additional accounts.
You don't have to worry about it, it just works in the background making more content available to your current Lemmy account.