Schema.org (often called Schema) is a semantic vocabulary of tags (or microdata) that you can add to your HTML to improve the way search engines read and represent your page in SERPs.
Schema.org is the result of collaboration between Google, Bing, Yandex, and Yahoo! to help you provide the information their search engines need to understand your content and provide the best search results possible at this time. Adding Schema markup to your HTML improves the way your page displays in SERPs by enhancing the rich snippets that are displayed beneath the page title.
For example, the first search result above contains both a star rating and a publication date. Both of these can be added using Schema. The second example does not have rich snippets and instead displays either the meta description or other information chosen by Google. To get the review rich snippet, you would use the following code:
<div itemprop="aggregateRating" itemscope itemtype="https://schema.org/AggregateRating">
<span itemprop="ratingValue">[Aggregate rating given]</span> stars –
<span itemprop="reviewCount">[Number of reviews]</span> reviews
</div>