Fleep username is your unique identifier in the Fleep network. It is made up of the user's chosen name and the domain @fleep.io. For example a user named Casey Platt can choose the Fleep username email@example.com.
By default, all users are prompted to choose a Fleep username when they are signing up to Fleep. You can see your username in the bottom left corner of the user interface on the desktop version of Fleep and in your account Settings:
Can I change my Fleep username?
You cannot edit or change your Fleep username once it's been set. (If you have a compelling reason though, contact us at firstname.lastname@example.org and we'll see what we can do.)
P.S. While setting a Fleep username is currently a mandatory step during sign up - it wasn't always so. So, there are some users who do not have a Fleep username. If you don't have a username then other users cannot @mention you.