SEO (Search Engine Optimization) involves increasing traffic to a website through organic search results. It describes how search engines like Google discover, index, and rank websites. Google uses crawlers like Googlebot to fetch pages, read them, and add them to an index where they are described and located. A site may be missed if it is not well connected to other sites, new and not crawled yet, difficult for crawlers to read, causes crawling errors, or blocks crawlers through its policies.