In plain language
Click/tap targets must be at least 24x24 CSS pixels, or have enough empty space around them.
Who it affects
People with tremor or larger touch contact areas, and anyone tapping on a moving vehicle.
How to check
Measure small controls (icons, dots, x-buttons) with dev tools: under 24px is a fail unless spacing or a bigger duplicate control compensates. Worked example — A used-car marketplace's listing filter: measure a checkbox/radio's actual clickable hit area: if it's limited to just the 13-16px native glyph and the adjacent label text isn't part of the clickable target, it fails.
Common misconception
A checkbox's visible square can look 24px while its actual clickable area, set by CSS, is much smaller. This rule measures the real hit target, not the visual size. Real example: on a subscription box service's account settings, the toggle switches in account settings render at 16 by 16 pixels with no spacing between them, well under the 24-by-24-pixel minimum target size the criterion requires.