It is used to describe the process designing your website in a way that it will appear higher in search engine rankings. Search engines often turn a blind eye to what you might assume are some of the web's most attractive pages. Just think of the tools many designers use to make web pages exciting. Great illustrations. Stunning photos. Animation and sweet music. Chuck them all onto your page and the world's most important web 'spiders' might well crawl right past.
In general, the earlier (or higher on the page), and more frequently a site appears in the search results list, the more visitors it will receive from the search engine. SEO may target different kinds of search, including image search, local search, video search and industry-specific vertical search engines.