In a recent video update on youtube, Matt Cutts, the head of the webspam team at Google, explains what is referrer spam, how it happens and what should be done about it.
For starters, referrer spam is done by making repeated requests to websites by making use of a fake referrer URL that points to the website/video that the spammer wishes to advertise. Referrer spam is targeted towards manipulating the search engines. With this technique the spammer gets a free link which results in a better search engine rank for his website.
In this video Matt Cutts explains how a spammer makes use of referrer spam to increase the ranking of his website/video. But, he also mentions that's the spammer may not be the website/video owner, but just a random person monkeying around with the referrer header to bring bad publicity to the website/video owner.Matt Cutts Throws Light On Referrer Spam And How To Deal With It!,