What is the FloatingActionButton widget?

FloatingActionButton (FAB) is a circular button that floats above the main content, representing the primary action of the screen. It is placed in the floatingActionButton slot of a Scaffold and automatically positions itself. It requires a child (usually an Icon) and an onPressed callback. Use FloatingActionButton.extended() for a wider button with both an icon and a label. Only use one FAB per screen.