SEARCH ENGINE (GOOGLE , BING, ALTA VISTA etc) + OPTIMIZATION (MAKING IT perfect for)
Eg: You want your website to be optimized for the search engine in such a way that it is ranked in the first page.
The best way to learn SEO is to learn the SEO PERIODIC TABLE given by searchengineland
Source: https://searchengineland.com/guide/what-is-seo