Structured data markup is used for search engines to create rich snippets. Schema markup helps search engines to determine the important information on a page. It defines the relationship between different types of data on the page.
This data has replaced meta tags to format the data crawled by Google. It does not affect ranking but can have significant effects on the success of your listing as it provides additional information to catch the attention of the visitor.
You can use the Structured Data Testing tool set up by Google to see if your site currently has any structured data and also to see if any additions you make are registering correctly.
A great place to learn a bit more about all of this and the vocabularies used to support structured data across many applications is the Schema.org website. The ‘Getting Started’ area of the site provides a very comprehensive guide of how to get started and how to integrate (especially if you are not using the Structured Data support tool from Google) the markup best.