Schema Markup
Also known as: structured data, JSON-LD
Structured data in a page telling machines what the page is about, in a vocabulary they already understand.
Schema markup, usually JSON-LD, describes a page's meaning in a standard vocabulary: this is an article, this is a question and its answer, this is a defined term.
Generated from the same structured content the page renders from, so the two cannot drift apart.
Schema does not guarantee a rich result or a citation. It makes a page legible and eligible. Anyone promising specific placements from adding schema is describing something they do not control.