<x id="START_TAG_STRONG" ctype="x-strong" equiv-text="<strong>"/>Check your email<x id="CLOSE_TAG_STRONG" ctype="x-strong" equiv-text="</strong>"/> to validate your account and complete your registration request.
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.