Below is a very simplified definition of Schema.org markup.
Schema.org markup is a meta markup that you can use to add fine tuned details about the content on your website. With the additional shema meta data, you can strengthen your site's SERP ranking
It is a way to communicate more effectively with Google's search engine algorithm by essentially labeling each piece of data, telling Google exactly what the data represents. A human can comprehend what an address or a video are, but to Google, it's all just code. But with schema markup, Google can actually understand what it's looking at. There are a number of categories.
Here is a quick example of what one might do with the schema.org markup
<h1>The Red Man</h1>
<span>Director: Josh Rose (born April 18, 1979)</span>
<h1 itemprop="name">The Red Man</h1>
<div itemprop="director" itemscope itemtype="http://schema.org/Person">
Director: <span itemprop="name">Josh Rose</span> (born <span itemprop="birthDate">April 18, 1979)</span>
<span itemprop="genre">Super Natural/Horror/Fantasy</span>
<a href="https://www.youtube.com/watch?v=tEaa3AGmJFA" itemprop="trailer">Trailer</a>
As you can see, we are breaking down each indivdual piece of data and labeling it for the search engines to digest. With this extra, helpful labeling, your website should do better in its rankings on the search engines.