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.
Some of your channels are not fully set up. Make them welcoming and explicit about what you publish by adding a <x id="START_TAG_STRONG" ctype="x-strong" equiv-text="<strong>"/>banner<x id="CLOSE_TAG_STRONG" ctype="x-strong" equiv-text="</strong>"/>, an <x id="START_TAG_STRONG" ctype="x-strong" equiv-text="<strong>"/>avatar<x id="CLOSE_TAG_STRONG" ctype="x-strong" equiv-text="</strong>"/> and a <x id="START_TAG_STRONG" ctype="x-strong" equiv-text="<strong>"/>description<x id="CLOSE_TAG_STRONG" ctype="x-strong" equiv-text="</strong>"/>.