VoidCallBack

This is a way for us to pass methods through the constructor of widgets/methods. But what is it really? Well, diving into the definition of it in the documentation we can see that it's only a typedef of void Function().

A typedef is just a type alias. It lets us do things such as

typedef IntList = List<int>;
IntList il = [1, 2, 3];

If you want to learn more about the different dart features give the language tour a read in the dart docs!