What is the Fediverse?
The Fediverse is a massive social network made of smaller social networks that can communicate with each other. This is possible due to a common protocol used by its members, called ActivityPub. Technologies that implement the ActivityPub include Mastodon, Lemmy, Peertube, Pixelfed, Threads, among several others. Social media websites created with these technologies can communicate with each other, i.e. they're part of the fediverse. This means, for example, that you can follow someone on Threads from a Mastodon account.
The Fediverse is often synonymous with decentralized social media, but it doesn't mean the same thing. For example, Bluesky is a decentralized social media technology that uses a different protocol, meaning that two Bluesky instances can communicate with each other, but they can't communicate with the Fediverse, so you would have a Fediverse and a separate "Blue-verse" in this case.