Search Engine Optimization,SEO is a technique to manage your content in a way that it becomes search engine friendly. In other words lets say you sport a cell phone blog, you should populate blogposts/content with high quality searched keywords. This will enable the search engines to route traffic to your website when keywords (like Cell Phone,iPhone etc for a cell phone website) are searched. However this is just a basic level definition. There's a bigger picture to the story when we talk about increasing Page Rank through SEO and generating traffic.
Search engine optimization (SEO) is the process of improving the volume or quality of traffic to a web site from search engines via "natural" ("organic" or "algorithmic") search results. Typically, the earlier a site appears in the search results list, the more visitors it will receive from the search engine. SEO may target different kinds of search, including image search, local search, and industry-specific vertical search engines. This gives a web site web presence.
SEO is search enginge optimization. explanation see http://en.wikipedia.org/wiki/Search_engine_optimization