My background is a system admin (yes, a big career change if I'm accepted into nursing school
) with a focus on security.
A super user by definition is someone who has been given the authority (typically via specific user / login credentials) to have (usually) complete access to the system.
Typically a person with super user privileges has two login credentials -- regular user (sometimes with escalating privileges often referred to as "power user") and super user.
The idea is that you should only be logged into the system as super user to perform specific tasks requiring those privileges; and, then be logged in as a regular user to perform every day days not requiring super user rights.
In my experience, a super user should have an in-depth understanding and working knowledge of the operating system, the applications installed, and the network / hardware infrastructure.
Failure to have that knowledge on an very through level often leads to security holes, a compromised system (security and working).