In plain language
At 320px width (400% zoom on desktop) content must reflow into one column with no horizontal scrolling and nothing missing.
Who it affects
Low-vision users at high zoom and people reading on narrow mobile viewports.
How to check
Set the viewport to 320px wide (or zoom desktop to 400%): scroll only vertically; confirm no content or controls have vanished. Worked example — A streaming music app's search results: set the viewport to 320px and try to open hover-dependent desktop menus: if the hover target becomes unreachable at small width with no tap alternative, it fails.
Common misconception
Having a “mobile view” isn't the same as passing reflow. This rule is tested by zooming a desktop browser to 400%, not by shrinking the viewport, and the two don't always behave the same. Real example: on a social media platform's settings menu, when the page reflows at small width / high zoom, some content or controls simply disappear with no other way to reach them.