Structured data
Also known as: schema markup, schema.org, rich results markup
Machine-readable statements embedded in a page that declare what it is and what the things on it are, rather than leaving a parser to infer it from prose.
In practice this means Schema.org vocabulary expressed as JSON-LD in a script tag. A page can declare that it is a WebPage, that it is about an Article, that the Article has an author who is a Person, and that the Person is the same entity as a profile elsewhere.
Without it, every fact about the page has to be inferred, and inference is where an answer engine gets you wrong.
Structured data does not make a page rank. It makes a page legible. Those are different claims and conflating them is how the topic got its reputation for snake oil.
The failure mode worth watching is markup that describes a page more impressively than the page deserves. The whole premise is that machines eventually check. Structure that looks good at a glance and falls apart on inspection converts your best argument into evidence against you.