Schema markup (structured data) is special code added to a page that helps a search engine understand its content: that it is a recipe, a product, an event or a review. Thanks to it, a result in Google can be richer, for example with a star rating, price or photo (rich results), which usually raises click-through. It is a technical but worthwhile element of SEO: it does not guarantee a higher position, but it makes the result stand out in the list. (Compare: Metadata, SEO)