On page SEO or search engine optimisation is making sure that your website is as search engine friendly as possible. If your website is not optimised then you have less chance of getting good results in the search engines, here is a quick guide towards good on page SEO:
Make sure that all of your web pages can be indexed by search engines - make sure that they all have at least one link from somewhere on your site.
Make sure that you have unique content on every single page.
Make sure that your meta-tags are arranged correctly - your page title tags and description tags should describe the content of your different web pages. The page title tags should be less then 68 characters and the description tags more detailed but less then 148 characters.
Make sure you label the different headers on your web pages using H tags.
Make sure that your web page URLs are SEO friendly, use mod re-write for Linux and Apahche hosting