Through the universe management page, you can create a new universe, add a description for it, and add existing stories or series to it. You can also delete existing empty universes.
Once a universe is created, it will appear in the submission wizard when you're submitting a new story. You can select to link or not to link the submitted story to a universe. If you choose to link it, then the story will appear on the site with a link to the universe in its description.
You can always add stories or series to a universe or remove them from it.
Each item in the universe, whether a story or a series, can have an optional little blurb, separate from the item's own description, that describes how the item fits within the universe.
Do NOT create a universe for just one story. When you start with a second story in the universe, then you create the universe and add the initial story and the new one once it's posted.
A universe can be set as public (openly shared - any author can post stories in it), or Multi-author (public-restricted — Only authorized authors can post in it) or private (only its creator can add stories to it).