Schema markup
Structured data written with the Schema.org vocabulary — the shared type system (Article, Product, FAQPage) search engines understand.
Schema markup is structured data that uses Schema.org — the shared vocabulary, backed by Google, Microsoft and others, that defines types like Article, Product, Organization and FAQPage and the properties each one carries. It is what you fill in when you add JSON-LD to a page.
The mistake is describing something the page doesn't actually show, or shipping markup that fails validation. Match the type to what the page really is, and keep it passing Google's Rich Results test.