Read more about linkbacks in vBulletin forums here:
vBulletin SEO Forums - LinkBacks
Simplified told, it is a mechanism telling you someone was referring to your thread on another website, or linking the thread on an external page (i.e. in a forum on another website). You can then view the referrers at the bottom of the thread - the other website may contain relate information that may be of interest for you too.
