Schema markup (also called structured data) is a standardized vocabulary, maintained at schema.org, that labels the parts of your content in a way search engines can parse directly. Instead of Google guessing that a number on your page is a price, schema markup tells it explicitly: this is a Product, this is its price, this is its availability.

The most common implementation format today is JSON-LD — a block of code added to a page's HTML that doesn't change how the page looks to visitors but gives search engines (and increasingly, AI search tools) a clear, structured read of the content.