SEO, which stands for search engine optimization, works to increase the visibility of a website on search engines like Google, Yahoo, and Bing. By optimizing your site in a way that search engines recognize it as authoritative on a particular topic or subject, you website will rank higher in search results.
For example, if your website is about the popular TV show Friends, then you want your site to rank highly in Google when someone searches for "Friends" or "TV Shows 90s."