Administrator is a user role in WordPress. When a user installs WordPress, it creates a new user with the username and password defined during the installation. That first user is assigned the user role of administrator. They can perform all actions on a WordPress website and have full capabilities.
A user with administrator role can also add and remove other users with the same role. When assigning the administrator role to a user, it is important to remember that administrators have the ability to delete content.
Administrator is the only user role that has the ability to upgrade a WordPress blog. They can change themes and edit core WordPress files by using the built in theme editor. They also have the ability to add, delete, and modify any plugins on the the site in the same way.
In most cases there is only one administrator. In the case of a WordPress multisite installation, some of the capabilities of the admin role are assigned to the super admin role. The super admin could modify themes, add new users, add and remove plugins, and administer the site network while the roles of the admin would only be concerned with managing a single site.