It's like Twitter, except anyone can create an instance and they can all connect to each other or only specific other instances or none at all. It essentially means that censorship is impossible because there's no single person or entity in charge of it. Here's a slightly outdated video that explains how it works: https://www.youtube.com/watch?v=t1lYU5CA40o (It's outdated because nobody uses GNU/Social anymore and all the cool kids have migrated to Pleroma. Avoid Mastodon. It's faggots.)