A supergrader is a grader with elevated privileges. Specifically, a supergrader can view and edit any submission within a course. Whereas a typical grader can only be assigned submissions or claim from the Submission Queue.
Supergraders have access to the following screen within the Grader Console.
Why should I make a grader a supergrader?
Supergrader status allows admins to grant graders elevated privilege without granting them the full scope of admin privilege. For example, a supergrader can view and edit any submission, but cannot edit a course's roster, create new assignments, or publish existing ones.
Users often designate the following types of course staff as supergraders:
Course staff involved in auditing grader work
An admin can grant a grader supergrader privileges by navigating to the Roster tab of the Admin Console.