uid 1 super user - what is is for
The Drupal user with uid = 1 is created by Drupal during the installation process of Drupal. User 1 bypasses all access callbacks in the system by default -- he can see/do anything.

Beyond that it is a typical Drupal user. If you want to secure the "admin" or uid=1 user please refer to this documentation page: Securing user #1 on steps you can take to secure the usage of the uid=1 user.