Hello Jose
In the Designer View of App Inventor, you arrange the required components such as images, lists, buttons etc. That view is fairly representative of what the App will look like on the device (a phone for example). You can also view your app on a connected device, or in App Inventor's Emulator.
If you invest a day working on the free App Inventor tutorials from MIT, it will save a lot of headaches and grey hair!
There are links on my website: