The Fediverse is a group of social networking sites that "federate" with one another. That means if you're on Mastodon (the Fediverse's Twitter), you can follow people who are on Pixelfed (the Fediverse's Instagram)! For more information, see this website.
To participate, you need to find an instance. An "instance" is one of the sites on the Fediverse. It's like an email provider, i.e. Gmail. It becomes part of your longer form username. My Instance is s.b252.gq. Yours might be mastodon.social, pixelfed.xyz, or peertube.video. Yet you can still follow me and I can still follow you. To find an instance that's right for you, check out this quiz.
Finally, you can just check out my account right now by going to my profile. Once you're on an instance of your own, you can follow me from there by using the Remote Follow button!