Search Engine Optimization (SEO) is a collection of marketing and site development best practices that help improve your position in organic search engine results. If you own a website, everything that is considered SEO is all stuff you should be doing anyway, because, well, you own a website.
So, what are the benefits? Well, SEO helps improve your position in organic search engine results. A better position in organic search engine results can (not will) give you a better chance at driving more visitors to your website. More visitors to your website could (not will) give you more business… and so on.