Search engine optimization (SEO) is a step-by-step process to improve the visibility and quality of a web page or website for users on a web search engine. It is also known as "organic" results for users' search queries. It is a set of methods to increase your search engine ranking position and traffic to your websites.
In other terms:
Search Engine Optimization is the method of increasing the quantity and quality of traffic to your website in organic search engine results.
How SEO works?
Type a question into a search box and Google, Yahoo!, Bing, or whatever search engine you're using magically replies with a long list of links to web pages that could probably answer your question.