The SEO factors you control on the page itself: title, headings, content, internal links.
On-page SEO is everything you can change in the HTML of a page: the title tag, meta description, headings (H1, H2, H3), body content, image alt text, internal links to other pages. These signal to Google what the page is about and how it should be understood. Good on-page = the basics most SEO is built on.
On-page is the cheapest part of SEO, you don't need external action, just editing. Most sites have significant on-page improvements available that would lift rankings by themselves. Audit yearly; fix weekly.