Search engine optimization (SEO) is the practice of getting targeted traffic to a website from a search engine’s organic rankings. Common tasks associated with SEO include creating high-quality content, optimizing content around specific keywords, and building backlinks.
In other words:
SEO is all about improving a site’s rankings in the organic (non-paid) section of the search results.
The main benefit of ranking for a specific keyword is that you can get “free” traffic to your site, month after month.