What people call “the Fediverse” is a collection of web applications that talk to each other through an open standard called ActivityPub. ActivityPub defines what users, groups, posts, comments and likes are, how you can subscribe to them and how they travel between instances. People have built different software packages that all use ActivityPub but have different user interfaces to feel similar to different “traditional” platforms. Mastodon is like Twitter, Mastodon and kbin are similar to Reddit, PeerTube is similar to YouTube, Pixelfed is similar to Instagram or Flickr and Plume is a long form blogging platform similar to Medium or older versions of Wordpress. Because they all use the same protocol under the hood, they can generally talk to each other. The user experience isn’t great yet but you can already use your Mastodon account to post to a lemmy community or to comment on a Plume post. Imagine it a bit like email where Gmail’s web interface, MS Outlook, Thunderbird and dozens of other clients exist as well as several different Mail servers. They can all talk to each other even though they were written by different people and all have their own interpretation and extensions to the SMTP and IMAP standards that define how emails work
Threads is a new microblogging Application by Meta (Facebook / Instagram) that will probably work very similar to Mastodon. In contrast to most other fediverse applications, Threads won’t be open source but will still use ActivityPub so it will be able to talk to existing open source applications. People here are afraid that they will abuse that to spy on people or systematically archive everything that happens in the fediverse in order to sell your data or train AI with it. They propose that we defederate from Threads (meaning we block our instances from talking to Threads’ instance). My post contains my thoughts on why that isn’t as useful as people think it is.
Hope that helps. If you still have questions, I’m happy to answer them to the best of my knowledge.