Scaffold is, as you may guess, a widget. It is usually recommended to use a Scaffold widget as the root widget for your screen, as you have in this recipe, but it is not required. You generally use a Scaffold widget when you want to create a screen. Widgets that do not begin with Scaffold are intended to be components used to compose screens.
Scaffolds are also aware of your device's metrics. AppBar will render differently depending on whether you are on iOS or Android! These are known as platform-aware widgets. When you add an app bar and you run your app on iOS, AppBar formats itself to avoid the iPhone's notch. If you run the app on an iOS device that doesn't have a notch, like the iPhone 8 or an iPad, the extra spacing added for the notch is automatically removed.
There also other tools in a scaffold that we will cover in the next chapters.