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.
Key is a type used in Flutter to identify widgets and allows Flutter to know when a widget that's moved in the tree is the same as a widget that was previously in a different location. There's a good video about keys here: https://www.youtube.com/watch?v=kn0EOS-ZiIc
