On page Optimization are the changes you make to your site physically, this includes optimizing Html Design Elements, optimizing internal link structure, writing good contents that suits the site needs. The major On-Page Factors of Search Engine optimization are:
Meta Tags (some engines)
Keywords Words in the Title Tag
Keyword Density in the content
Keyword Density in each Page
Words in Headings
Words in Bold
Keywords in URL
Anchor text links
HTML Validation
Listings in directory
Beginning Words
Link Structure
Hundreds of Other Factors
Why on page factor is that much important ?
Your site has to rank. If your site is not ranking for the targeted keywords then conversion rate of your site will be less. The ultimate goal to run a site is to have business. To make conversion rate of a site higher, on page factor is important. So to have a best quality site is to have an on page factor optimization.
Meta Tags (some engines) 
