Schema markup (structured data) is code, usually JSON-LD, that labels the content of a page using the shared vocabulary at schema.org: this is an article, here's its author and date; this is a product, here's its price.
Why it matters
It removes guesswork for search engines and AI systems, and certain types can make your result eligible for richer displays in search. It doesn't guarantee better rankings, but it helps machines read your pages accurately.
Example
A blog post can include BlogPosting markup with its headline, dates and publisher, so search engines know exactly what the page is and when it was updated.
What to do
Add markup that matches what's visibly on the page, keep it accurate as the page changes, and check it with Google's Rich Results Test.