The Fediverse is 'the federated universe', which is a network of interconnected software. "Federated" means that even if we have accounts on different servers, we can still follow each other, talk to each other, etc. Email is federated, so a Gmail user can email a Yahoo user, for example. By contrast, Facebook is not federated, so I'd need a Facebook account to communicate with others on Facebook.
So, in this vast network, we don't just have Mastodon, an alternative to Twitter. There are alternatives to blogging sites like Tumblr; alternatives to image sharing sites like Flickr or Instagram; and they can all follow and talk to each other.
If you feel lost, here's a video that explains in clear language and has visual aids.