Well, Search Engine Optimization is tuning your site to make the wise and best use of search engines. Basically, its all about working the kinks of your site to make search engines recognize your contents and serve them better to people.
Some of SE0, best practices include content building, link building either through guest commenting, guest blogging, or just about anyway to get do follow links back to your site. However, note that content(original) building is the best way to get steady organic traffic to your site. And link building is best done via a higher rating site to a lower rating site.