The Fediverse (often shortened to ‘Fedi’) is a collection of social networks that communicate (‘federate’) with each other, rather than being isolated like the majority of popular social media clients like Facebook and Twitter/š. One of the more common examples of software that people use to interact with The Fediverse is Mastodon, but there are many others.
Participation in The Fediverse has an unfair reputation for being more complicated than in more traditional, billionaire-owned social media options. I suspect that this is at least in part because the billionaires have more resources to spend on making people think that Mastodon (etc) are more complicated and have a vested interest in putting people off using them.