What is fediverse, which Meta's Threads has joined?

The fediverse is an open network of interconnected social media servers which allows people to communicate across different platforms. Meta's social media app Threads has joined fediverse, which includes apps like Bluesky, Mastodon and Flipboard. If a Threads user turns on sharing to fediverse, people on other fediverse servers like Flipboard will be able to see their posts.