Everything you control on the page
SEO splits into two halves: what happens on your pages and what happens off them. On-page optimization is the first half — every signal you directly control within a page's own content and code. Off-page factors like link building are about your reputation across the web; on-page is about making each page itself as clear and relevant as possible.
The core idea is simple: help both the search engine and the human understand, instantly, what this page is about and why it deserves attention. When you get on-page right, Google can confidently match your page to the right searches, and visitors who land there immediately find what they came for. Everything else in SEO gets easier once your pages actually say what they mean.