SEO is the process of increasing the amount of visitors to a web site by ranking 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 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.
The web pages showing the results of a search engine's search for keywords and pages on which those keywords appear. SERPs for any keyword search may number in the hundreds or thousands. Internet users usually only read the first few SERPs. Websites that appear on the first few SERPs have the highest chance of being accessed by the searcher. If your site is accessed more, you will earn more from the service or products on your site.