Structured data, written as a JSON island in your HTML
JSON-LD (JavaScript Object Notation for Linked Data) is the way Google recommends you tell search engines what your page is about in machine-readable form. You add a <script type="application/ld+json"> block to your <head> with a JSON object describing the page.
When a search engine finds valid structured data for a supported type — like Article, Product, Recipe, or BreadcrumbList — it can render a Rich Result: review stars, prices, event dates, and clickable breadcrumbs in the search results.