I don't necessarily have the best understanding either, but I'll give it a shot.
The fediverse is a federated universe, which means lots of servers can talk to each other (like a federation of people) instead of all being centralized in one company and their servers. It's like how email is a common way to talk to each other, even though there are different groups with different versions.
Lemmy is the Reddit like portion of the fediverse. There are other parts, such as Mastodon, which is like Twitter. These can talk to each other a bit too, but right now they mostly talk within themselves.
Servers like vbin, sh.itjust.works, Lemmy world, Lemmy.ml, and many others are computers that store the information on Lemmy. They keep track of how many upvotes, text, links, and sometimes content hosting. But because there are a lot of them, and anyone can easily make their own, it's harder for a company like Reddit to just ruin it all for profit.
randomperson