Onpage optimization is optimizing your your website through the content of your site. Like on site title, description, keywords, articles etc
Never use copy content on your own site or from another site. Each page needs to be distinctive! When duplicate content is used, the site may be penalized and perhaps removed from search engines.
On page means add various types of meta tags that send strong signal to SE about particular page. Write relevant unique and descriptive ccontent for each pages.
On page optimization is the first thing which you have to do after completing your websites. There are a lot of on pages activities like Meta tags update, URL structure according to your services, Heading tags, Relevant and unique content (Use Bold, Italic, Underlines for Keyword), proper navigation, call to actions, use breadcrumbs, internal linking etc.
These things need to be done on on-page:
1 Keywords Analysis
2 Meta tags
3 Alt tags and Heading tags
4 Analytic code
5 Xml and html sitemap
6. Robots.txt files
7. 301 redirect code
8. Google webmaster tools Verification
On page basically comprises of title, keyword, description, header tags, image optimization, sitemap etc.,