Specifically what is SEO?
Search engine optimization (SEO) is the practice of increasing the amount and quality of visitors your website through organic and natural search engine results.
What goes into SEO?
To understand what SEO really means, discussing break that sentence down and look at the parts:
Quality of traffic. You can attract all the visitors in the world, but once they're approaching to your site because Google tells them most likely a resource for Apple computers when really most likely a farmer selling oatmeal, that is not quality traffic. Instead you want to attract visitors who are genuinely considering products that you offer.
Variety of traffic. Once you have the right people clicking through from those search engine pages (SERPs), more traffic is more preferable.
Organic results. Ads constitute a significant portion of many Search page results. Organic traffic is any traffic that you no longer have to pay for.