Content SEO is creating content with the intent of ranking in the search engine results. When a user searches for something related to your content, you want it to show on the first page of the results. Content SEO includes the content you write as well as the way it is structured on your webpage.
Search engine algorithms are somewhat of an enigma. While not many know the exact formula for ranking at the top of the first page of results, there are experts who have analyzed top-ranking pages to determine what factors cause a piece to rank on the first page of Google.