Structured data is a block of machine readable facts you add to a page so an engine can read it as data, not just prose. The common format is JSON-LD, a small script in the head that describes the page using the shared vocabulary at schema.org.
It tells an engine what the page is: an article with this headline and author, a product with this price and rating, a set of questions and answers, an event with this date. Engines use it for rich results in search and, more and more, to trust and lift facts into AI answers.
Two rules keep it useful. Use the right type with its required properties: an Article needs a headline, a Product needs a name, an FAQPage needs its questions. And the markup must match the visible page. Structured data that claims things not on the page is treated as spam and can earn a manual action.
Topkay validates the JSON-LD on a page, checks the required properties, and flags where the markup and the visible content disagree. Fix-it generates a starting block for a page that has none.