HTTPS is “secure HTTP”. Though using HTTPS isn’t a must for forums, but recommended. Forum members when they try to log in using a password if the site isn’t over an HTTPS, the browser throws a warning to the user.
SSL
How to Switch An Existing XenForo Setup to Secured SSL (HTTPS)
A step by step guide to switching an existing XenForo installation from HTTP to HTTPS aka from http://domainname to https://domainname