Structured data is a standardised format for giving Google information about a page and classifying what is on it. On a recipe page, for example, it can label the ingredients and the cooking time, so Google does not have to work them out from the text alone.
Most structured data used by Google Search uses a shared vocabulary called schema.org. It is usually written as JSON-LD, a block of code placed in the page. Visitors do not see it; Google reads it.
This guide shows you how to choose something Google supports, add it, test it, and check how Google sees the page.