Creating good, worthwhile content is a key part of SEO. If you write, or create, good, worthwhile content, you are taking part in SEO. If you take your time to write a headline that best reflects your article, and mark it up correctly, that's a part SEO. If your site is using easy to read names, that's SEO. If people link to your content and write about it, that's a part SEO. If you make the description below your page title on Google's search results actually worthwhile then some random piece, that's a part SEO.