SEO or Search Engine Optimization (Optimized or Optimizer- google calls it Search Engine Optimizer) is the process of design or re-design of pages of a website to the most searched keywords & phrases and implementation of a bunch of other aspects so that the automated programs of search engines are most attracted to pick up these pages and present them at the top when such keywords or phrases are searched.
The whole subject of SEO has come into light because search engines are robots and these robots use artificial intelligence to take decisions. During the process they are programmed to crawl and index the WebPages on the Internet in a systematic way.
Systems can be understood and therefore with proper SEO techniques websites can be designed to fair better in search engines.
The concept of SEO is more often complex than said. It encapsulates, constantly changing and ever-increasing number of methodologies to make the WebPages fair better with search engines.
The end result of a SEO web page or website is that it starts getting crawled from search engines and starts getting listed on top during search results. Therefore the site ends up with good relevant traffic.