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.
What Is: Config.php
Config.php file resides inside the library folder and is used to configure XenForo settings.
What is: Web Server
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.
What is: localhost?
The localhost means this computer. The IP Address (IPv4) for the localhost is 127.0.0.1 and in (IPv6) it is ::1.
How To Set Custom HTML Pages as Homepage or Index Page in XenForo
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.