Your subdomain on the web!
When initially creating a site using Universe on the Web, you're asked to choose a username before you can sign up for an account.
A Universe username is a subdomain where you site initially lives on the internet. You're able to change this in the Universe app for iOS and macOS devices.
All subdomains appear on the internet as username.onuniverse.com.
Are you trying to log into the Universe App to edit your site? You'll just need to choose sign-in and use the same method to log-in that you used when creating your site for Universe on the web.