SEO (Search Engine Optimization) refers to the collection of techniques and practices that allow a site to get more traffic from search engines (Google, Yahoo etc). SEO can be divided into two main areas: on-page SEO (website changes to make your website rank better) and off-page SEO (work that takes place separate from the website). Remember, a website is not fully optimized for search engines unless it employs both on and off-page SEO.
During searching a website by any search engine, if site comes at last in the sites listing, you can use various technique of SEO to show your site in 1st position in the listing that increase traffic and clicking of the site.