As an administrator, you will find pages permissions in “Pages” page. It allows to define permissions for each page, to let users access pages based on their permissions.
Pages can be made “private” (as opposed to “public”) to let non-logged users access them or not. For example, the “sample.php” page can be made public if you want to share samples with non-registered people. A page is “public” when “private” option is unchecked.
When opening a specific page’s permissions, an administrator can choose what users can see the page (the others are redirected to their account). This can be useful to restrict some pages to administrators, or other groups of users.