H1
The single top-level heading that states what a page is about.
What it means
One per page. Zero leaves the subject unstated and forces everything reading the page to infer it. Several make the machine choose, and it will not always choose the one you meant.
The part a short definition leaves out
Multiple H1s are almost never an author decision. They come from templates that wrap the logo in an H1, or a section component that uses H1 for its own title. If a page has five, look at the template, not the content.
Where this comes from
See whether this is costing you anything
The inspection reports on this signal directly, alongside thirty-seven others, and tells you whether it is a quick fix on your site or something deeper.