The abbreviation “SEO” stands for “Search Engine Optimization”. It gathers all the techniques and strategies available to bring a given website on top of the search engine result pages (SERP) for a given set of target keywords.
It’s more of a dark science really, so don’t believe any gurus swearing they can bring your url on top of Google (provided you pay a sizeable amount of dollars the size of the Himalaya) : the techniques they’ll use will most probably get your site banned by Search Engines like Google or Bing. The industry actually designates them as “black hat seo”, versus, of course, the “white hat seo”. White hat SEO will only play by the rules set by SE providers: produce valid and semantic html code, write good content, properly use the title tags (title and headers) and so forth…