SEO stands for (Search Engine Optimization). Its is provide of getting more traffic unique, organic and nature, free in the all search engine. SEO is the technique to make show your website on search engine .99% people use search engine like Google, Yahoo and Bing this is primary search engine all country.
On page SEO is most important on your website. In the on page seo you use proper per page optimize title, meta tag like (keyword, and description).
Off page SEO is most important your website it is fetch traffic other websites and from social network. In off page seo you can do like link building, increasing link popularity by submitting open directories etc.