Heading Hierarchy
Heading levels used in descending order so the page has a readable structure rather than just styled text.
What it means
H1, then H2s for sections, then H3s inside them. Skipping from H2 straight to H4 breaks the outline a retrieval system builds when it tries to work out which part of the page answers a specific question.
The part a short definition leaves out
The usual cause is picking a heading level because of how it looked. Use levels for hierarchy and CSS for size. That is the entire repair.
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.