In plain language
Use real text, not pictures of text (except logos or where the exact look is essential).
Who it affects
Low-vision users who need scalable text, and people who customize fonts or colors.
How to check
Try to select text with the cursor — if headings, buttons, menus, or paragraphs can't be selected, they're probably images of text. Worked example — A grocery delivery app's shopping cart: try to select the content of a 'screenshot' (release notes, quotes, error messages): if it's used as the primary content instead of real text, it fails.
Common misconception
Teams reach for an image when they want a specific font or exact visual effect. But if the visual look can be done in CSS, it should be, images of text can't be resized, searched, or restyled. Real example: on a music streaming app's playlist screen, a promotional banner renders its headline and price as a single flattened image instead of live text, so the text can't be resized, recolored, or reflowed by the user.