I'll try to simplify some concepts about the Fediverse and Lemmy using some more common IT knowledge.

I assume you are familiar with emails: gmail, protonmail, hotmail, aol, each of them runs a Server which will give you access, when registered, to a mailbox which will have a name of your choice, followed by an AT(@), and then a Domain specific to the vendor you have chosen.

ID

Lemmy works in a similar way: When you choose an instance (for example lemmy.world, or lemmy.ml, or feddit.de, you can find a list here: https://lemmyverse.net ) you are choosing where your account will live. You'll get a full ID composed by your name of choice followed by an @ and then the domain of your chosen instance.

In your case your full ID is favrion@lemmy.world . This is unique to you in the whole Fediverse.

Communities

Contrary to emails we have another abstraction: Communities. They are collections of posts and comments, moderated by someone and about a specific subject. These communities will reside on a specific server, like your account, and you can, for example, go to !technology@lemmy.ml or !mildlyinfuriating@lemmy.world . As you can see their format is similar to that of your account: they have both a community name, the name before the @, and a domain that specifies where the main copy of it physically resides.

You can find a full, searchable, list of communities here: https://lemmyverse.net/communities For some more complex reasons just disregards any of them that reside on beehaw.org (they have defederated with lemmy.world, but you don't need to understand this concept yet).

Feeds

When you start to look around the lemmy.world site, and you are logged in with your lemmy.world account, you'll have the ability to follow three different types of feed: Subscribed, Local, All.

Local: This feed shows only communities which main copy actually resides on the instance that you are currently on.
All: All the posts from all the communities of all the instances will appear here.
Subscribed: All the posts from the communities that you have subscribed to will show here.

Federation

Until now the concepts were pretty straightforward and not so dissimilar from other services, but now starts the more complex subject: Federation.

For all this different instances (computers running the Lemmy server) to have the information it needs to spread and they need to duplicate it.

This will allow you to see the same exact content whichever instance your account is on: this means that if you have chosen to create an account on lemmy.world or lemmy.ml it, generally speaking, doesn't matter. You will see and interact with the same content and people.

Your instance is just the gate towards the content, which you have to always pass through (so no, you shouldn't make an account in each instance, you just need one, and you should choose an instance which rules and values are acceptable for you and, better yet, similar to yours).

Going back to the previous comparison with email services you can think of this as: Each time you take an action (writing a post, a comment, sending an upvote) you are really just sending an email to all the instances, so they get notified and update their copy of the content.

An email saying something along the lines of "favrion@lemmy.world has just downvoted the post number 69420 in the community 42 of the instance 123" will propagate until every single server has received it.

Defederation

Defederation is the strength and the weakness of the whole fediverse: it helps to moderate the content but it also breaks the web connecting all the servers.

Each instance automatically connects itself with all the others present in the net, but an Admin can actively decide that some other instance - maybe because it is full of Bots, or Scammers, or contains communities clearly voted to illegal or immoral things, or just they don't like it; the reasons are subjective - really rustled their jimmies and they don't want to see it anymore. In that case the Admin could decide to Defederate from them: this means that each time a new email from that instance comes it just get dropped and ignored.

So, if Instance A defederates from instance B, each time B sends an update to the content produced by the communities or people from B it will just be ignored by A. All the users registered on A will just not see anything or anyone from B.

Basically an instance wide shadow-ban.

Conclusion

This was just a quick and rough explanation of the concepts, but I hope it was useful: if you have any question I'll try to answer them at the best of my abilities. But keep in mind that they are not really high. And I'm kinda tipsy.

The main takeaways are this:

• You only need an account in a single instance.
• You can subscribe to any community that you want, wherever they are.
• You can search instances and communities here: https://lemmyverse.net
• You should choose an instance that has similar values as you.
• Sometimes the federation (the mails sent around between the servers) can get slow, but all content will be eventually consistent. Kinda.
• Communities are sprouting around at high speed, there are a lot of duplicates and everything is in flux, try to be patience.
• Just try to have fun and be happy.

EDIT: Typos.