These Primary Groups dicate the privilages granted to a user by Phylobyte CMS. Although various plugins and extensions can add more fine-grained group and permission management, this simple system will be sufficient for most general needs.
The admin group allows access to Phylobyte CMS. Do NOT delete the admin group, or remove all users from the admin group, or you can not access Phylobyte CMS.
You likely won't want to do much with the goups here, they will mostly be created by other plugins. You should use this tool as a sort of reference as you create new users and logins to give people the permissions you want.