Website SEO or Search Engine Optimization in its simplest form is about getting the word out, to the right people, that your website exists.
“If you build it, they will come.”, might have worked for Kevin Costner's character in Field of Dreams, but that kind of thinking can equal failure for a website. Unless you are creating a website for your own pleasure and don't care if anyone will ever see it you will need to let the world know. And the world uses search engines!
So...What is SEO and how do you do SEO?
SEO involves techniques to increase a website's ranking in search engine results, so that the website appears closer to the top of the list when relevant terms are searched. The closer to the top of the list, the more visitors a website will have from the search engine users.