Structured markup is the term given for the standardised code added to HTML code that gives important definitions for the elements that feature on webpages. It’s a vital addition, as it is the code that Google and other browsers will use to index webpages for relevant SERP Features.
For instance, if structured markup is added to an online recipe, it’s data that Google can retrieve from a webpage’s HTML code to display that recipe in a SERP Feature when web users are searching for such a recipe.
Google offers a Structured Data Markup Helper tool to help webmasters create this important code for their webpages. The Schema Markup Generator also helps webmasters produce the code.