Have you tried reading any of the (many, many) articles posted right here in this forum on the topic of search engine optimization? Or any of the (literally) hundreds of thousands of SEO-related articles posted elsewhere online?
On page refers to the content of the page. Obviously, the content on the page is vital to SEO. Without content, the search engines have nothing to index.
You're going to have to narrow down your question before anyone can give you anything approaching a meaningful answer. What specifically do you not understand about content and its relationship to search engine rankings?