On-page SEO refers to the practice of optimizing many elements on a webpage or website which are taken into account by search engines to attribute a rank on SERPs.
The term on-page SEO is used as opposed (or rather as a complement) to off-page SEO (backlinks strategy).
On-page SEO is optimizing all or most factors used by search engines. Google takes into account more than one hundred on-page criteria.
Some of the on-page factors:
Some of on-page SEO techniques are black hat techniques.