Think like this. On reddit, you had old.reddit.com and www.reddit.com, here you have lemmy.world, kbin.social, and many more.
On reddit you have (I making the names up): /r/politics, /r/politicaldiscussion /r/truepolitics etc. All about the same, but slightly different. Here you may have politics@lemmy.ml politics@kbin.social and many more.
The rest goes behind the scene - fedeverse decentralized, run on many different servers, but you can simply ignore all that, and think about all of that as names and themes.