The web is full of billions of web sites.  Search engines such as Google, Yahoo and MSN allow the users of the web to sift through this large amount of data in order to find specifically what they are looking for.  If your site is not listed in the first page of search results when potential visitors search for your content, then your potentially missing out on large amounts of traffic.

Search engine optimization (SEO) is the process of increasing the amount of visitors to a Web site by ensuring that the site ranks high in the search results of a search engine. The higher a Web site ranks in the results of a search, the greater the chance that that site will be visited by a user. It is common practice for Internet users to not click through pages and pages of search results, so where a site ranks in a search is essential for directing more traffic toward the site.

SEO helps to ensure that a site is accessible to a search engine and improves the chances that the site will be found by the search engine. It also aids the search engines in knowing which page to return for a particular search phrase.