super is used to call the constructor of the base class. So in your example, the constructor of CardTitle is calling the constructor of StatelessWidget.
Keys are used as an identifier for Widgets, Elements, and SemanticsNodes.You don't need to use Keys most of the time, the framework handles it for you and uses them internally to differentiate between widgets. For more on them see: https://flutter.dev/docs/development/ui/widgets-intro#keys