Mastodon is one of a newer crop of decentralized social networks that are trying to put the users first instead of building everything around ads, tracking, and attention-grabbing feedback loops.
It’s open source software run on a bunch of smaller networks, mostly volunteer-run, each with its own policies and moderators. (You can even run one yourself if you want!) These smaller networks can talk to each other behind the scenes, allowing you to follow and interact with people across networks (that’s where “federation” comes in). Each instance has a different community and flavor, and each is part of the broader community.
I’ve been on for a few months now. It takes a while to settle in, and it feels kind of like the early, optimistic days of Twitter, plus there’s an effort to learn from the past and make it better this time. You can find me at @email@example.com (general discussion) and @firstname.lastname@example.org (photography).
If you’re interested in trying it out, joinmastodon.org is a good place to start, and can suggest which instances might be a good fit. And this Brief Introduction to Mastodon goes into a bit more detail on how to use it and what the culture and etiquette is like.