The What's This...? button provides help windows with the intention that the user wants to get help about a certain widget within the working view or a toolbar item. It is placed in the toolbar and gets activated once the user hits the button. The cursor changes to an arrow cursor with a question mark like the button itself looks like. The the user can press on a visible widget item and gets a help window. As an exercise, you could try this behavior with the What's this...? button within KDevelop.

To add the What's This...? help to one of your widgets, use the static method QWhatsThis::add(QWidget *widget, const QString &text)