Definition
Structured data is a standardized format for describing a page and classifying its content, usually written with the Schema.org vocabulary. Google supports JSON-LD, Microdata and RDFa, and recommends JSON-LD in most cases because it is easiest to implement and maintain.
Why it matters for your business: structured data helps search engines understand exactly what your business, services and pages are, and can make pages eligible for richer search results.