Search engine optimization, or SEO as it is often shortened to, is a method and work process that connects information searches on search engines to companies and organizations. When the searcher trusts you that you will solve their problems, they choose to click on your search result and then lands on your web page.
Search engine optimization is an economically profitable traffic source because it connects the demand (search) with the supply (your service). Therefore, SEO as a traffic source often tends to be one of the best options in terms of each invested dollar.