Structured data, also called schema markup, is a type of code that makes it easier for search engines to crawl, organize, and display your content.
Search engines will look at your website and see content. To users, we can look at content and understand which part is a product name, price, size, author, etc. Search engines need a little more help seeing those categories. Structured data is a universally agreed to syntax that search engines look for to understand what the content on a webpage means
Let’s say you want to provide contact information for your company. You could just add the information onto your website footer; human visitors will see that and immediately know that is your contact information. But search engines will have to work a little to figure out what that raw data means.
However, if you use schema markup, you can tell search engines that your contact information is your contact information (instead of just some random data). You can specify which part is the street name, the ZIP/Postal code, phone numbers, email addresses, etc.
There are currently 829 types of schema, ranging from local business schema to takeout menu schema. Whilst this seems like a scary prospect, you are very unlikely to use all 829 types on your website. You will most likely use 4-5 of them. Don’t worry about having to code either; Wordpress has many plugins to help you add structured data to your website. If you have a custom coded site, there are still many resources that you can use to add structured data to your site.
If you are unsure if you have structured data on a webpage, Google has a useful tool that can check for you – aptly called the Structured Data Testing Tool.