The accordion widget is a vertically stacked list of items represented via buttons. By clicking on a button, the item can be "expanded" or "stretched" to reveal the content associated with that item. Within each accordion widget there is always exactly one item "active". When a new item is stretched, the previously active item shrinks back to only show its button.
The accordion widget is similar in purpose to a notebook widget. The accordion widget is useful when dealing with long item descriptions because in that case you quickly run out of horizontal space on a notebook widget.
Since the accordion widget is so similar to a notebook widget, the API is kept as close as possible to that of the ttk::notebook widget. See the manual page for details.