According to Moz, SEO (Search Engine Optimization) is:

 

 the practice of increasing the quantity and quality of traffic to your website through organic search engine results.”

Quality of traffic. It’s about being targeted. Having the right people clicking the website who have demonstrated an interest in the site’s content (product or service).

The quantity of traffic. Once it’s the right people, the more the better.

Organic results. People trust the organic search results more than they do the paid ads. You get more clicks and better conversions. Most importantly, a better ROI.