The administration panel can only by accessed by logged-in users, and depending on the user role of the user, he/she might not be able to access all parts of the administration panel. A user with the subscriber role, for instance, is only able to view and edit his/her own profile page.
The opposite of the back-end of a website is the front-end, which refers to the parts of a WordPress site that are publicly accessible. That includes its front page, sub pages, blog archive, blog posts, and so on.
Back-end can also be used as part of the job title “back-end developer”, which refers to a developer who is responsible for the core functional logic of an application, website or service. That includes communicating with the database to retrieve data, manipulating that data, and preparing the data for output on the site.