A username (also known as a subdomain) is where your site lives on the web!
When initially creating a site using Univer.se, you're asked to choose a username before you can sign up for an account.
A Univer.se username is a subdomain where you site initially lives on the internet. You're able to change this in the Univer.se app for iOS and macOS devices.
All subdomains appear on the internet as username.univer.se.
Are you trying to log into the Univer.se App to edit your site? You'll just need to choose sign-in in the top right hand corner, then use the same method to log-in that you used when creating your site for Univer.se on the web.