SEO stands for "search engine optimisation." It is the process of getting traffic from the "free," "organic," "editorial" or "natural" listings on search engines. All major search engines such as Google, Yahoo and Bing have such results, where web pages and other content such as videos or local listings are shown and ranked based on what the search engine considers most relevant to users. Payment isn't involved, as it is with paid search ads.
New to SEO? Why not take three minutes and watch our video introduction to search engine optimization? It'll quickly cover the basics: