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 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. The higher a website naturally ranks inorganic results of a search, the greater the chance that that site will be visited by a user.
SEO is typically a set of "white hat" best practices that webmasters and Web content producers follow to help them achieve a better ranking in search engine results. Search Engine Land's Guide To SEO explains the ranking factors in more depth, in a tutorial providing tips and advice on implementing them.