A disclosure widget is a button that reveals or hides an adjacent block of content when activated, commonly seen in accordions, expandable FAQ answers, and read more sections. The trigger needs a real button element (or an equivalent role), an aria-expanded state that reflects whether the content is showing, and a programmatic connection to the content it controls.
The native HTML details and summary elements provide this pattern automatically and are the most robust choice when the design allows for it, since they need little custom scripting to behave correctly.