...is what tells the site where the person came from to get to the site. This is obviously useful to help people optimize their pages to get more people. Pr0n spammers are now using this to spoof their site and just meaninglessly crawl thousands of sites to distribute their referral header on the thousands of ignorant "webmasters" that use open to public, online statistics software they just configured and installed.
The process of spoofing is pretty simple even for non programmers. There's free spoofing extensions for firefox that you just have to install, and it'll look like there's a ton of people from x site whenever you browse their site continually. All the programmers have to do is make a very basic crawler and add the appropriate referrer header.
If many people do this, which they already are, it creates a mess. The stats are no longer really valid as there wont be away besides actually making a crawler yourself to see if that site is actually linking you. This is possible however it would also be a big pain in the butt if you have to do it for all 30,000 links you've gotten.
It may be of value to people who consider another page seeing which site they come from an invasion of privacy, but among webmasters this is pretty standard practice and helps us a lot to provide optimized content for those same users. Most people dont know enough about computers to even know the referrer field exists, but as the new generation learns more and more and extensions are made easily available for quick download, it's going to more and more difficult to really say who came from where.