Schema markup is a special code added to the website's HTML that provides search engines with additional information about the content on the page. This information makes it easier for search engines to understand the content, resulting in better visibility and indexing. In the case of Magento SEO, schema markup helps search engines better understand the products and categories present on the site, which can lead to better rankings and visibility in search results. Additionally, it can improve the appearance of search results with rich snippets, showcasing product information, reviews, and other details that can attract potential customers. Overall, schema markup is an essential component of Magento SEO that can significantly impact a website's visibility, traffic, and conversion rates.