The referrer check ensuresthat people are posting from your forums url.
Disable Referrer check turns the check off completely.
Domains for Referrer Check simply allows you to specify which domains are yours to allow in the check...
Multi URL access is for telling the forum to allow URLs which aren't your primary:
This means say, i have undergroundnews.com undergroundnews.net undergroundnews.org; where my primary url is set as undergroundnews.com with multi url access I can still access and login to undergroundnews.net and .org and stay logged into those domains; without multi-url access I will be redirected to undergroundnews.com
