In plain language
All functionality available with a pointer must also be operable through a keyboard interface without requiring specific timings for individual keystrokes.
Who it affects
Blind users, people with motor disabilities, and anyone operating the interface without a pointer.
How to check
Unplug the mouse: Tab/Shift+Tab through the page, activate with Enter/Space, operate widgets with arrows — every feature must work. Worked example — A pharmacy app's prescription-refill form: tab through video player controls: if any control (play, seek, volume) can't be reached or activated by keyboard, it fails.
Common misconception
Passing a Tab-key sweep is necessary but not sufficient. Full keyboard operability also means every control activates with Enter or Space, and no custom widget silently depends on a mouse-only event. Real example: on a bank's mobile money-transfer screen, a feature only responds to mouse-specific events (mousedown, hover, gestures) — keyboard users cannot trigger it at all.