SEO involves optimizing a website to increase its visibility in organic search results. This is done through on-page techniques like choosing descriptive titles and URLs, as well as off-page techniques like getting other sites to link to the website. Search engines use web crawlers called spiders to discover and index websites by following links and analyzing page content. Key on-page optimization techniques include using targeted keywords, adding metadata tags, structuring content and links well, and regularly updating the site. SEO results should be measured over time to track progress.