This section displays a preview of what your App will look like when displayed on a mobile device. You can use the scroll bar to view from top to bottom.
The visual design is not a working copy of the App, but is interactive so you can click on items to modify them. You can drag and drop photos to rearrange them and click on various sections of the app to display the settings dialog for that section. This is similar to clicking on the cog for a section in the Layouts Section area of the builder.
To produce a working copy, you need to publish your App.