Administrator is the one who have all the control in WordPress website. It is created while installing WordPress, when the user creates username and password during the installation that user is assign as a role of administrator.
A user with a Administrator role can only change the themes and WordPress core as well as install and uninstall WordPress plugins. Administrator can assign the role to other users with the limitation. It can also assign other users with the same role that have ability to change WordPress core as well as edit and delete a content.