Come one come all to the Lemmy-verse! It's nice and cozy here, we do have some "bad parts of town" but you can do an instance block and not deal with them lol
For your memes we have !memes@lemmy.world if you like sciency posts mander.xyz has some excellent communities (communities=subreddits) like !science@mander.xyz and !biology@mander.xyz and for a meme science combo theres always the fantastic !science_memes@mander.xyz
You can also drop a shit(post) off at !lemmyshitpost@lemmy.world or hang out at !onehundredninetysix@lemmy.blahaj.zone (though you do have to post before you leave that one!)
There's much more around as well!
Obligatory, fuck Spez
Roughly, yes. Imagine there isn't just one reddit, but multiple reddits, all with their own users, sub-reddits and eventually even rules. Now all these multiple reddits can "talk" to each other. That's why I'm on lemmy.zip and I can see a post someone hast posted on lemm.ee - that is federation. Saved on different servers, but looks like one coherent feed.
Defederation means that an admin of an instance excludes one or more instances from communicating with his. I'm not 100% sure what every implication is, but roughly, the instances can't communicate with each other, so if lemmy.world defederates from lemmy.zip, for example, we could not have this conversation right now. You also would not see any posts from lemmy.zip and I would not see any from lemmy.world
I see, thank you very much!