SEO means Search Engine Optimization and is how you write your text and describe your pages so search engine can find you and rank you more highly than others. Google has by far the largest market share and there are trends to follow that will boost your standing. These techniques change over time so it's best to check your ranking from time to time on your keywords.
How Google decides on where to place you on its free listing is unknown but there are a number of tools and techniques that can be used to help your rates. YWS will implement on your site these techniques plus tracking tools to see how your rank rises over time.