Referer spam is kind of old school but because of that, many SEOs don't fully understand what it is. Referer spam is basically making repeated requests to a specific web page with the goal of showing up often in the referer logs and analytics. If web sites publish their referer logs, then that link in the referer logs is indexable by search engines. If not, webmasters may notice the spike in traffic, visit the web page and maybe think it is important to link to it.
Matt Cutts, Google's head of web spam, explained all of this in a short two-minute video:
Hopefully that explains an old school tactic in a nice concise manner.
Forum discussion at Google+.