In plain language
Headings and labels must describe their content/purpose.
Who it affects
Everyone, especially screen-reader users and people who skim headings/labels for orientation.
How to check
Read all headings and field labels out of context: does each one still tell you what it's about?. Worked example — A dating app's profile setup: check for duplicate labels used for different purposes (e.g., 'Name' for both a product field and a user field) within the same form: ambiguity here fails.
Common misconception
Vague section headers like “Overview” or “Details” are technically headings, but they fail this rule because they don't tell a screen-reader user, jumping heading to heading, what's actually in that section. Real example: on a music streaming app's playlist screen, playlist controls are labeled with generic text like "Button" and "Item" instead of descriptive labels such as "Shuffle playlist" or "Add to queue," so the label doesn't describe what the control does.