Technically the software has a feature which, when someone e-mails a link to a thread here (that "E-Mail This Page To A Friend" button), it sends a number as an identifier. That number is the number that corresponds to the user's 'identifier' number in the database. That 'identifier' number is, I believe, the next number in sequence from whoever registered just before you did. So - every user has a unique 'identifier' number.
So - you e-mail a page to someone and they click the link in the e-mail which opens their browser and brings then to the page. That number is on the end and the software recognizes it. It sees that user number (yours - you sent the page to someone) and 'credits' your 'account'.
I don't think I have it turned on - I can't remember. It wasn't a feature or statistic that I'm interested in.
In short, for here it's a useless extra feature. Some sites that use the software may use it to give people certain credits in some way but I've never really thought about how one might use it.
I'm sure you all can come up with a good reason though, so I'll check and see if I have it 'turned on'.