SEO is important because it helps people find your site online.
On page SEO is also referred to by many as “Traditional SEO”. On-page SEO focuses on what is actually on the page. The process of making sure your code is formatted properly, the target keyword exists on the page in the content, in the page headers, in the picture’s alt tags etc. For more information about the nitty gritty here’s a great Backlinko.com article about On Page SEO that goes into a great amount of detail. This form of optimization is the original kind of SEO, while it is still a requirement for a well-optimized site, there are so many other factors besides on-page SEO that is also in play these days that you must make sure you do not focus on your on-page SEO alone. A site that only has been optimized for on-page SEO will not be able to compete based without also being optimized for the other key factors.