Yoast SEO pour Magento is a plugin that helps optimize a Magento site for search engines. It provides features like XML sitemaps, meta tags, structured data, canonical URLs, and more. The presenter experienced issues with duplicate content on their Magento site and used Yoast SEO to address it through features like removing duplicate product buttons and adding rel="next" and rel="prev" tags for pagination. They emphasized the importance of monitoring Google Search Console and allowing time for optimizations to take effect after launching or updating a site.