Structured Data
Also known as: schema markup, schema.org markup, JSON-LD
Structured data is extra information you add to a page so search engines and AI know exactly what it is.
It says: this page is an article. This person wrote it. It was published on this date. It is about these topics.
Without it, they have to guess from your text. With it, you say it outright, and they get it right more often.
Most structured data uses Schema.org, a shared list of labels for almost everything on the web. Most sites add it as JSON-LD.