The fediverse is a decentralized network of various social platforms, typically connected together using the ActivityPub protocol.
The fediverse is unlike the modern idea of "social media", with the primary factor being that it is decentralized. This means anyone with the technical know-how can run their own peice of the fediverse, that can communicate with the rest of the it! These so-called "pieces" are known as instances.
While the fediverse has multiple kinds of platforms, they all are able to communicate with eachother, meaning an Mastodon instance and a Akkoma instance would still be able to communicate.