SEO is a technique that helps search engines find and rank your site higher than the millions of other sites in response to a search query. SEO thus helps you gain traffic from search engines.
Search engine optimisation or SEO is the simple activity of ensuring a website can be found in search engines for words and phrases relevant to what the site is offering. In many respects, it's simply quality control for websites. Having said that, if there was ever an industry that was little understood by 'outsiders' it's SEO.