At the beginning of the recipe, did you notice this line?
Theme.of(context).textTheme.headline6,
This is a very common Flutter design pattern known as "of-context," which is used to access data from other parts higher up the widget tree.
Every build method in every widget is provided with a BuildContext object, which is a very abstract sounding name. BuildContext, or context for short, can be thought of as a marker of your widget's location in the tree. This context can then be used to travel up and down the widget tree.
In this case, we're handing our context to the static Theme.of(context) method, which will then search up the tree to find the closest Theme widget. The theme has predetermined colors and text styles that can be added to your widgets so that they will have a consistent look in your app. This code is adding the global headline6 style to this text widget.
