In plain language
Keyboard focus must move through the page in an order that makes sense.
Who it affects
Keyboard and screen-reader users who follow focus rather than the pointer.
How to check
Tab through each page and each dialog: focus should follow the visual/logical flow, enter dialogs when they open, and return to the trigger when they close. Worked example — An online marketplace's seller dashboard: navigate to a new route in a single-page app: check where focus lands — jumping to the page top or an arbitrary widget instead of the new content's heading fails.
Common misconception
Visual order and DOM order can look identical to a sighted developer and still diverge completely for a screen reader, especially once CSS positioning or grid layout gets involved. Real example: on a news outlet's article page, tabindex (keyboard-focus setting) values force a tab order that jumps around illogically, so keyboard users move through the page in a confusing sequence.