Super Admin is a user role included on WordPress multisite (network) installations.
On multisites, users with the super admin user role are the only users that can add new sites and delete existing ones, and they also have complete access to every aspect of every site in the network. That means that they can edit all content and change all settings in the network.
With those capabilities in mind, it’s best to limit the number of super admins on a network to as few as possible. If a user needs access to content on multiple sites in the network, it’s better to add him or her as administrator (or an ever lower user role, like editor, if he/she only needs to edit content) to those specific sites.