What it is, and what it is not
Structured data is a block of JSON-LD in the page that states plainly what the page is about: this is an organisation, this is its address, this article was updated on this date, this product costs this much. It is a description, not an instruction — search engines are free to ignore it, and they frequently do.
- It does not make you rank higher. Google has never treated it as a ranking factor and still does not
- It cannot make a page eligible for something the content does not support — marking up a review that does not exist is a manual-action risk, not a shortcut
- JSON-LD in a script tag is the format to use; microdata scattered through markup is legacy and harder to keep correct
- It is machine-readable meaning: the same facts your page states in prose, restated where a parser cannot misread them
- Getting it wrong is usually worse than omitting it, because invalid markup is simply discarded and the effort is wasted