In many technical Drupal how-to articles, the author will refer to "Drupal User 1" or "the first Drupal user" or "UID 1" - in all cases, the author is referring to the initial user that was created when the site was first set up.
This first user is special. Very special. The first user set up on any Drupal installation has access to everything: all functions, permissions, and features enabled on the site. From a technical standpoint, the user with ID = 1 doesn't even have to pass through Drupal's access control system - it's like going to an airport and not having to go through security.
For this reason, it is often recommended that once the site goes live, this Drupal User 1 should only be used sparingly, only when necessary. It is advised that you create a secondary account for the main site administrator with a limited set of "everyday" permissions.