Structured Data Markup is the info you can specify in the code of a page so that Google could add a nice rich snippet to your page in the search results (this may be a star-rating, a location/working hours of your business, event details and so on).
You can just test any page in Google`s structured data testing tool and see the list of errors prioritized, you can also find a brief explanation of each error: for instance, in your markup you may be using a property that is not recognized by Google for an object of given type. Let`s say you have a mobile app and you have added a 'number of downloads' property, which is not available for the 'MobileApplication' object type and therefore cannot be recognized by Google - you`ll get the error in such case. The tool will even highlight the code lines involved in the error.