What is the WordPress admin page?

Admin page is the one where the owner of the blog (and the team who has the admin access/edit access) can login to the backend of the website and make required changes, approve posts/comments/content, modify the website etc.

It can be accessed only through a specific admin user ID and password

Adding up to the above comment, WP admin is the backbone of a Wordpress site. Anything can be added and removed as per the themes. Also the number of features you can use depends on the Plan you have bought. The access of the site is allowed as per the role you play for the site. Not every role needs a password.