On-site SEO (also known as on-page SEO) is the practice of optimizing elements on a website (as opposed to links elsewhere on the Internet and other external signals collectively known as "off-site SEO") in order to rank higher and earn more relevant traffic from search engines. On-site SEO refers to optimizing both the content and HTML source code of a page.
Beyond helping search engines interpret page content, proper on-site SEO also 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).

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References https://moz.com/learn/seo/on-site-seo