ELI5:

Kbin, Lemmy, mastodon,etc all speak the same language. That language is called activity pub.

Any app can be created and speak to all of the other apps as long as we all speak the same language.

All of these apps combined make up the concept that we call the Fediverse.

Reddthat uses the Lemmy application to interact with all of the other apps and servers.

Email is an equivalent system. Where anyone can talk to anyone else as long as they speak the same language.

Kbin and Lemmy are very similar but call items differently, communities/bins. They focus around following a community.
Mastodon speaks the same language but focuses around following people.

These differences make it hard to easily show the information coming from the other apps.
(This is also why Lemmy/kbin are having Federation problems at the moment)

Lemmy instances can communicate with each other. Kbin is a bit on the microblogging side of the fediverse.

Also I don't think it matters what the title is all of the Lemmy instances use the same protocol i.e. Activity Pub to communicate with each other. Just like the instances of Kbin do. I guess the user interaction with each other or post is different on Kbin and Lemmy.