The fediverse is a technology that lets users of independant services, such as social media and video hosting websites (à la YouTube), communicate without both users needing to be using the same service. This kind of technology has been in use for many years now: think of your telephone, and how even when you and your friends are using different telecom providers, you can still call eachother and send text messages. The same principle is behind the fediverse: being able to communicate, regardless of digital-borders.
The fediverse is actually fairly easy to join, the first step is to find a service (often refered to as an instance). There are many places where you can find these instances. My personal recommendation is to look at fedi.garden for instances that peak your interest; don't worry if you think you've made an incorrect decision later, the software your instance is running will almost certainly have the ability to export your user data in a format that lets you move to another instance (see Mastodon's guide, otherwise, check your user settings for an option like that).
Why the fediverse?
For lack of a better explaination, centralized (as in centered on a singular service) fully lead to vendor lock-in, which forces users to only ever stay on one of these services, with no choice to move, lest they lose access to all their friends or social groups. The fediverse prevents this by allowing users of different services to communicate without needing to create a new account on the other service, and better yet, allows you to easily move your account and user data if you dislike the actions of the staff on the service which you're using, or want to try other service for awhile.
Regarding Bluesky
I personally prefer the fediverse over Bluesky. I trust the federation systems there much more than I do the ones at Bluesky, and I don't feel this is unfounded: it took until 2024 Feb 22 (for reference, they officially released 2024 Feb 06) for them to release a proper way to store your own data, Bluesky has been in development since at least 2019, maybe 2021 if we're being generous; additionally, to my knowledge, there is still no way to host what they call "Relays," which essentially act as a layer to get your messages to everyone else (think of them as a postal office, the when you send a skeet, which is actually what Bluesky calls their posts, it goes through a relay to get to other users), what this means is that only the Bluesky team runs a functioning relay at the moment.
The fact that Bluesky released as a "decentralized" social media service without all their actually decentralizing features is particularly odd. Maybe when they get this sorted out I'll change my tune, but I don't know how long that will be.
Also, no, I don't hate Bluesky. It'd actually be quite odd if I did, since I actively use it. I'm just saying that I don't feel that it's decentralized. that's all.
Isn't decentralization bad?
I'm aware that most people think of decentralization as a bad word, and I know exactly why that is: cryptocurrency. Cryptocurrency has, for quite a long while, been seen as bad in many ways (of which I don't feel like getting into); because of this, it's mostly soiled the words "decentralized" and "crypto" (many even refer to cryptocurrency as just "crypto," and I just want to say this: crypto is short for cryptography not cryptocurrency, that is literally just short for cryptographic currency, and it's called that for reasons that do not matter to why it's bad).
I'm aware that's a bit rambly, but if you really don't like the word "decentralization," you can also call it federation (they're not exactly the same concepts, but for the purposes of the fediverse, they are).
Conclusion
I'm not sure if this will actually convince anyone to sign up for a fediverse instance, but I guess it was worth a try.
If this did convince you and you're looking for more information, I'd suggest looking at fedi.tips.