In plain language
Keyboard focus must never get stuck anywhere on the page.
Who it affects
All keyboard and screen-reader users who can become stuck inside a component.
How to check
Tab through the whole page including embeds and popups: confirm you can always Tab or Esc back out of every component. Worked example — A grocery delivery app's shopping cart: tab into a rich-text editor and press Tab: if it's captured for indentation with no documented alternate exit key (and none is advised on screen), it fails.
Common misconception
A modal that intentionally holds focus is fine, and expected. The failure is a modal that holds focus with *no* documented way out, no Esc, no visible close button. Real example: on an email client's compose window, keyboard focus can get stuck inside an embedded plugin/format (video player, widget) and the user cannot Tab back out to the rest of the page.