Structured data is on-page markup that is used to understand pieces of information used in web page content. Structured data allows you to annotate content to denote the context behind contact information, biographical information, company data or product data. Search engines are able to read this markup and better understand the relationship between and context behind the annotated data. Structured data is an important part of using the semantic web for SEO.
Structured data requires a taxonomy that search engines can understand – Schema.org. The most common way of adding schemas to pages is through JSON-LD code.