When you click on the Preview button (blue), you access preview for your application inside the app builder. You can quit preview by clicking on "Stop". You can also hide the topbar using the up arrow to have a full screen preview of your application. If the page you want to preview requires some parameters (if you have defined URL parameters ), a popup opens to provide the parameters for the page.
The preview of your application is only available to the application collaborators. To make your application available publicly, you can "Deploy" your application (view Next Section).
This preview is useful to view your application with real data and test dynamic interactions defined in the application builder. It is also useful to test the application with different screen sizes (desktop, tablet, mobile).
To test your app on different screen sizes, click on the breakpoints icon in the topbar:
Sometimes, based on the screen display selected, you might have to change the zoom level by using the zoom slider on the right.
When you are confident with your application, you can deploy it to make it available publicly.