These provide relevant contextual information and actions around a particular item. Callouts are generally used to show the user more information or actions about an item in a lightweight UI.
The SharePoint callout control provides a flexible way to engage your user and showcase your SharePoint-hosted app’s functionality. You can configure it in a variety of ways to suit your app’s UI.
When you do searches in a SharePoint 2013 site, you’ll see examples of the callout control in action, as it pops up whenever you hover over a search result. Figure 1 shows the callout for a single search result and shows a few of the typical things in a content control: a title, some information about the item on the page, and actions (Open and Send) that you can take on the item. In this case, the information and actions are relatively simple, but you can already see two advantages to using it. First, it lets you show additional information about elements in a page when that’s needed, and second, it gives you an elegant way of adding functionality to the page.