Rich cards are a mobile search result format that presents a carousel of search results in card form, i.e. a white rectangle on the SERP's grey background. Rich card search results include a title, an image, reviews (if they exist), and other info such as preparation time and calories (for recipes). In order for content to appear in rich cards, websites must implement schema.org structured markup on their web pages.
For more information on rich cards and other SERP features