Structured data, most commonly written using the schema.org vocabulary in JSON-LD format, is a standardized way of explicitly labeling what different pieces of content on a page mean: this is a product, this is its price, this is a review, this is the author, rather than leaving a search engine to infer meaning from plain text and layout alone.
That explicit labeling is what makes rich results possible, star ratings, prices, FAQ dropdowns, event details, showing up directly in search listings, and it's also part of how search engines build the underlying knowledge graph connecting entities across the web. There are hundreds of schema types covering everything from recipes to job postings to local businesses, but only a handful are relevant to any given page; the goal is marking up what's genuinely on the page accurately, not adding every schema type available in hopes something sticks. Google's Rich Results Test tool validates markup and shows which rich result types a page is eligible for, which is the standard way to confirm structured data is actually implemented correctly before assuming it's working.
