A superuser is a special user account for general system administration such as in networks and databases. It is a variation of the administrator user, which varies depending on the operating system and context. Most commonly, superuser refers to the most privileged user account in the Android mobile operating system, which can be activated by rooting the device.
Techopedia explains Superuser
Superuser is the term given to the user account that has overall control of the Android operating system and is usually achieved by rooting the device or the OS. Rooting means unlocking the OS restrictions to the user so that he/she may be able to make changes that are otherwise restricted by the OS. In other operating systems, this is known as administrator/admin (Windows) and root (Unix/Linux).
Superuser is also the name of an application used in Android that manages what other apps can do to a rooted device. It does this by giving that specific app superuser status as well.