The username is a unique identifier of your account on this and all the other instances. It's as unique as an email address, which makes it easy for other people to interact with it.
If you need help to use PeerTube, you can have a look at the <x id="START_LINK" ctype="x-a" equiv-text="<a href="https://docs.joinpeertube.org/use-setup-account" target="_blank" rel="noopener noreferrer">"/>documentation<x id="CLOSE_LINK" ctype="x-a" equiv-text="</a>"/>.