On-site SEO is optimizing elements on a website in order to rank higher and earn more relevant traffic from search engines. On-site technical SEO refers to optimizing both the content and HTML source code of a page.
Beyond helping search engines interpret page content, on-site SEO helps users quickly and clearly understand what a page is about and whether it addresses their search query. In essence, good on-site SEO helps search engines understand what a human would see (and what value they would get) if they visited a page, so that search engines can reliably serve up what human visitors would consider high-quality content about a particular search query (keyword).