That's fair. You don't need to know any of those things to use the platforms in whatever ways best suit you.

That said:

Decentralized means no one entity "owns" Lemmy. Lemmy is free, libre, open-source software (FLOSS). This means the software (Lemmy itself) is free to use, with few restrictions (or none), and available to anyone for review and/or modification. It's also built by contributors and any member of the public can offer contributions. Most open-source projects are like this.

This is the opposite approach of the social media platforms like Facebook and such. They use proprietary software that they claim to write themselves and claim to exclusively own. Each of the big tech social media sites is owned and operated by a company. They are centralized platforms. They have a center of control and users of those platforms are bound to the whims of the owners (CEO, shareholders, etc).

Choosing to abandon those platforms and instead use social media built using FLOSS (which no one person or group owns) means no one can stop you from having and using that platform however you want. It puts the power of social media in everyone's hands instead of only the hands of the rich and exploitative.

If the Lemmy instance you use gets shut down for some reason you can just go to another instance and keep using Lemmy.

Decentralization doesn't solve every problem though. A decentralized service can still be used to create some kind of monolith. If you wanted to create your own Lemmy instance and share really important information there then you could do that. You could also try to gatekeep it in some ways. Force users to do something you want or give you something of value in order to get the important thing you have. It would be dumb to try but you could still do it.

Federation means everything gets shared by default. If I post on infosec.pub (my current home instance) that information does not stay on infosec.pub. It is automatically offered by default to every other Lemmy instance that wants it. Administrators are able to change the default settings and many do so to curate their instance in ways that are helpful.

Federation means unity that people choose. Examples would be The United Federation of Planet from Star Trek or the European Union (kind of). The "Fediverse" refers to all the different software platforms that use the ActivityPub protocol to Federate with each other. Mastodon, PieFed, Lemmy, and a bunch of other software depend on ActivityPub to make sure the information users share is shared with everyone.

Decentralization makes it tough to gate-keep things (among other benefits). Federation makes it tough to manipulate or weaponize. Bad actors still try to do those things but using software like Lemmy or Mastodon for evil looks weird to everyone. Even for people who don't know what being decentralized and federated means.

I hope that info is what you're looking for and that you find it helpful. If you've got other questions feel free to drop a reply. 🙂

The info above is an overview. There's nuance I'm ignoring on purpose that some might be nitpicky about. Those people are probably correct, are also likely abysmal educators, and certainly no fun at parties.