aria-expanded is applied to the trigger element of a collapsible component, an accordion header, a dropdown menu button, a mobile navigation toggle, and must be kept in sync with the actual visual state. When the section is closed the value is false; when open, it becomes true, letting a screen reader announce collapsed or expanded automatically.
A common failure is toggling the visual content correctly with JavaScript while forgetting to update the aria-expanded value, leaving screen reader users with an announcement that contradicts what a sighted user sees.