Structured Data
Also called: Schema markup, JSON-LD
Structured data is machine-readable markup embedded in a page that states explicitly what its content means, most commonly using schema.org vocabulary in JSON-LD format.
Without it, a machine must infer from prose that a page describes a business, a product or an event. With it, the page says so directly: this is an Organization, this is its name, this is where it operates, these are its profiles elsewhere.
For answer engines the value is disambiguation. Structured data removes the guesswork about what an entity is and how it relates to others, which is precisely the guesswork that causes a business to be skipped.
Why it matters
It is the cheapest available intervention with the most reliable effect. Most sites either omit it or implement a fragment, which makes complete, accurate markup a genuine differentiator.