The administrator is a user role in WordPress. The administrator can access the backend of the website and has complete access to the website.
The administrator user role is registered when the core installation of WordPress. At the most initial installation of WordPress into hosting, it asks for an email and password that is used as an administrator of the website. admin can assign user roles to others and have complete access to all sections of the website.
Before assigning someone as the admin keep in mind that the admin can remove all roles of users and content.
Admin can change the themes, plugins, and core version of WordPress. Admin can also change the core files if he has access to core files, It gives admins the ability to add, delete, and modify any plugins on the site in the same way.
In the case of WordPress Multisite Network, All the permissions of admin are transferred to super admin. And sub-site admins have limited access over their network site but have complete access over their subsite.