Mastodon is a decentralized social network with tons of instances hosted arround the world.
Every instance has its unique set of users with their toots and whom they follow. All toots of your instance's users are shown in the Local Timeline. While all toots of people you follow are shown in the Home timeline (this is the timeline mostly resembles the original twitter one).
The Federated Timeline is kind of the combination of both for all users of your instance. So it contains all (public/listed) toots (+ boosts) of users and the onses the instance's users are following. So to say it's everything public the instances knows of.
The bigger and more connective the users of an instance are, the more populated is the Federated timeline.
@sparky8 hmm.. Im not totally sure what you mean with combo federation universe - but probably the right.
The feediverse is kind of the sum of all instances. Like each instance seen as a planet and you're living/moving on one instance but be aware that there's more actively hop between instances or know how to find your way arround.