Structured data is markup. Usually JSON-LD following the Schema.org vocabulary, that explicitly labels what things on your page are: this is a Product with this price and availability; this is an Organization; this is an FAQPage with these questions and answers.
Humans infer all this from layout and context. Machines shouldn't have to guess, and structured data means they don't. It turns a wall of text into labelled, extractable facts an engine can lift cleanly into an answer, which is why it consistently helps both classic rich results and AI citations.
For AI visibility the high-value types are Product/Offer (for commerce), FAQPage (direct Q&A engines love to quote), Organization and DefinedTerm (entity clarity), and BreadcrumbList. It pairs with llms.txt: structured data makes individual pages legible; llms.txt points crawlers at the right ones. Both make your facts easy to ground and cite.