A SuperUser is a user with the permission to do wiki-wide operations like:
Use despam action to mass revert edits of some user (see HelpOnSpam).
Use the SystemAdmin macro (see HelpOnMacros).
Use SystemInfo macro (see HelpOnMacros), showing more details than for a non-priviledged user.
Use WikiConfig macro (see HelpOnMacros), showing configuration details of the wiki.
Install software, install packages (see HelpOnPackageInstaller).
Install language packages via LanguageSetup.
In their settings screen, they have a list of other users they can temporarily "become" in order to e.g. help those users or even disable their account. Logging out returns to the superuser account (see HelpOnUserHandling).
Do not confuse a superuser with someone having the "admin" ACL right. The "admin" ACL right just means to be able to set/change/remove ACLs on some or all pages, while being superuser means to be able to do the above mentioned stuff - globally.
The people being superuser need to be much more trusted (than people with "admin" ACL right) due to this - usually the ones who install the wiki put themselves into the superuser list in the wiki config (see HelpOnConfiguration).
