What does defederation imply? Feeds aren’t aggregated, or users are not allowed from the other instance?

Iirc no activity from one instance is available on the other. No posts in their communities, no posts/comments from their users.

Also one other major question. I thought lemmy was its own thing, but I guess it’s part of the Fediverse? The Fediverse is just a set of protocols? What is lemmy then?

The fediverse is a group of social media platforms that use the active pub protocol to pass along user interactions between instances (servers run by different people).

Lemmy is the reddit of the fediverse, meaning posts in communities and comment in a trees structure.

Kbin is also the reddit of the fediverse, but with some less features (manly on the moderation side). Because they both use active pub in a reddit style they can easily interoperate.

Mastodon is the twitter of the fediverse meaning their content (mircoblogging) doesn’t fit the lemmy format but you will only sometimes see some posts like ‘If you can read this I have managed to post from mastodon to lemmy’.