"Structured Data" is a standardized format for presenting the search engine crawlers with additional information about a web page to assist them with indexing your website. The details can be found at https://schema.org/. You may have noticed Google now has 20 different formats for displaying search results. Many of these formats contain more information than what was provided by the traditional title and description meta tags. You guessed it, most of this comes from structured data placed in the page source code the same way the title and description are provided.
Our friends at moz.com have done an excellent job presenting a beginners guide to structured data https://moz.com/blog/structured-data-for-seo-1 You may also want to take a look at the source code of https://www.frankholt.com this is the absolute minimum that can be employed on an Accelerated Mobile Page (AMP). I need to add more tho this page when I get a chance. Google has an online test for your web pages. https://search.google.com/structured-data/testing-tool/u/0.
The simple way to create structured data for your web pages, and HTML emails, is to use the Structured Data Markup Helper provided by Google. https://support.google.com/webmasters/answer/3069489?hl=en&ref_topic=3068649 It will generate HTML you can paste on your pages.