SEO spam is search engine optimization spam. These are fake sites full of ads that make money when people visit them and click on the ads. The way they get people to visit is via SEO: tricking Google’s search engine to rank the site higher than other sites undeservedly. The way they pull off this trick is manifold (since there’s an arms race of techniques to detect SEO and new SEO that evades detection). Classic techniques include:

• filling the page with irrelevant keywords
• creating networks of sites that link to each other (link farms; this targets the PageRank algorithm which uses sites linking to each other as an honest signal of credibility and relevance)
• showing different content to the search engine’s crawler bots than you do to real users (cloaking)
• stealing content from popular sites (scraping) and using it to bulk up your fake sites (classic targets include Wikipedia, Stackoverflow, and tons of recipe blogs)
• tricking legitimate websites into linking to you (link farms tend to be detected and down-ranked en masse; legitimate sites linking into them help avoid this detection by reputation laundering)
• hacking people’s social media accounts to promote your fake websites
• hosting shady/illegal content such as porn, gambling, or TV and sports streams