Search Engine Optimization; or SEO, is the process of tailoring your website to increase visibility when people use search engines such as Google and Bing to find products and services. The more optimization that is done to a site and/or specific page can determine how much traffic that page is able to generate.
Google and other search engines use "bots" that will crawl each and every website for keywords, descriptions and tags, and then categorize them accordingly. For example; if you had a webpage about graphic design, and your site was optimized effectively, when somebody Googles "Graphic Design", your website will show up at the top of the list.
Without these keywords and tags, your site will be left in the dust. There are MILLIONS of websites on the world wide web, and your website will be buried underneath each and every one of them.