Structured data is machine-readable markup, usually schema.org vocabulary in JSON-LD, that tells crawlers exactly what a page describes: a product, its price, its availability, its reviews.
For commerce, the most useful types are Product (with Offer and, when reviews are real, AggregateRating), Organization, BreadcrumbList and FAQPage.
Structured data doesn't replace clear copy, but it removes ambiguity: an engine doesn't have to guess which number on the page is the price.