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.
I am at least 16 years old and agree to the <x id="START_LINK"/>Terms<x id="CLOSE_LINK"/><x id="START_TAG_NG_CONTAINER"/> and to the <x id="START_LINK_1"/>Code of Conduct<x id="CLOSE_LINK"/><x id="CLOSE_TAG_NG_CONTAINER"/> of this instance