Apps have an internal design, the programming that works with the user interface. Effective programming entails knowing how to use the internal components to effectively implement what is visible to the user.
A good way to think about the internal pieces of an app is to focus on components and actions. The components are the various tools you find in App Inventor to create tasks. Think about the Design Editor and all the onscreen tools: buttons, images, drawings, and so on. In the Design Editor, you pull into your app all the pieces that make up the user interface, or what the users of your app will see on the device’s screen.