In the HTML, label the email field "username", both name and ID attributes. Use "Email" as the label that the user sees. They will be setting a login that is an email.
Don't capture a separate username, as that might be confusing ("Which do I log in with?") Use Display Name as a handle that will be used throughout the site.
It’s referring to an HTML label tag.
This is the “right” way to mark up a label for a form field.
aria-labelledby is a work-around when the label can’t be marked up by a label tag (e.g. see this recent answer)
aria-label is an alternative when for whatever reason there isn’t a tag ...