As I understand it, it's a means of enabling folks to submit a particular post/thread from your forum to a site like Digg, or Del.icio.us or what have you. Once you turn it on, threads will feature a button or link that a user can click to automatically post it to those sites - sort of like the links you see at the end of this
Neatorama post (on the line that begins with "[Heart] this post?").
Personally I've never turned it on, but if you're looking for a way to get more publicity for your site I would imagine that it
could be useful ... ?