Search Engine Optimization focuses on organic search engine optimization or natural search engine optimization. The search engines are looking for natural link building to rank sites, but that natural link building doesn't happen as much as it did years ago.
The on-page optimization is extremely important. Your site should tell the search engines what it is about without in-bound links telling them. That is what On-page optimization does. On-page Optimization should be done before the search engines ever crawl the site for the first time.