Usergroups in XenForo has permission set only for the front end (inline moderation, edit post, ban users etc) and provides no access to the admin area.
Config.php file resides inside the library folder and is used to configure XenForo settings.
A Web server is a software that resides in a computer and serves the request from the user via a web browser to send them HTML data as output.
The localhost means this computer. The IP Address (IPv4) for the localhost is 127.0.0.1 and in (IPv6) it is ::1.
Forum list is by default the homepage or the index page for a XenForo installation but you can set any page as the index page or the homepage.