Super users (exists since version 1.5.2) are one or more trusted user names with permission to do wiki-wide admin operations, like make full backups, software installation, and language installation via SystemPagesSetup (do not confuse with HelpOnAccessControlLists).

In their UserPreferences screen, they have a list of other users they can temporarily "become" in order to help or disable those users.

See HelpOnConfiguration to configure the super user, and some examples in HelpOnUserHandling and HelpOnPackageInstaller.