SEO (search engine “optimization”) is how a search engine ranks its results. The more webpages link to a certain result (as determined by a webcrawler), the higher it is displayed. That is why bloggers are often paid by bad actors to publish editorials that link to a scam, virus, or gambling website.
Google popularized the concept in its early years, back when SEO was an organic indicator of a result’s popularity. It made them the single best choice. Then, capitalism happened, and SEO became a resource to exploit.