Structured data is machine-readable annotation embedded in a page's code — usually JSON-LD following the schema.org vocabulary — declaring what the content is: an article and its date, a product and its price, an FAQ, an organisation. Search engines use it for rich results; AI engines use it to extract facts with confidence.
The rule that pays: mark up what is genuinely on the page, never more. Our guide to schema markup that earns rich results covers the types worth having.