Search engine optimization (SEO) involves optimizing a website to increase its visibility in search engine results. There are three main types of SEO: on-page optimization which involves optimizing on-site elements under the website owner's control, off-page optimization which focuses on external ranking factors like backlinks, and technical SEO which improves technical aspects of the website like page speed and structure. Proper optimization of on-page elements like titles, meta descriptions, URLs, and headings can help improve search engine rankings.