What is Referral Spam?
Few webmaster inject their site URL in blogger stats of our blogs. So we can see their site as referring site in our stats. Just out of curiosity we click on the URL and they get the traffic. This kind of activity is called Referral spam.
What is Google doing about it?
Google knows this issue quite well. When they observe this kind of activity they block them. But webmaster change their behavior which Google blocking system fails to detect and they again start appearing in stats.