SEO stands for Search Engine Optimization. Which basically means 'how do I make my site as attractive as possible to Search Engines, so that my site ends up on the front page of Google and Bing'. SEO fills books (and thick ones at that), but a very very brief overview ...
Google loves fresh content and blogs. By adding to and updating your site regularly you improve the rankings.
Google loves video. Adding a video to the front page of your website can help you rise in the search rankings.
Google loves tags. By tagging your site, and adding the correct key words to your titles and content, you will help users find your site.
Of course that means that you first need to figure out what those key words. What will users search for when they find you? Talk to your target market, and look at Google analytics and Bing Analytics to see how they are currently finding you (more on that to follow).
Google loves incoming links that get clicked. So if otherweb sites reference your content, and their users click through to your site, that really helps you in the search rankings.