"Structured data markup" is a standard way to annotate your content so machines can understand it. When your web pages include structured data markup, Google (and other search engines) can use that data to index your content better, present it more prominently in search results, and surface it in new experiences like voice answers, maps, and Google Now.
Structured data markup makes your content eligible for two kinds of Google features:
Google's Knowledge Graph is a system for organizing information about millions of well-known "entities": people, places, and organizations in the real world. Google's algorithms merge information about entities from many data sources. However for some types of information, though, the best source of data is the entity itself.
Specifically, companies and people can now customize their own data in the Knowledge Graph by adding structured data markup to their official website. The following types of data may be customized:
To find out more about how to add structured data or customize your Knowledge graph check out the official Google Developers doucments.
Source: Google Developers