This handly little block is used for switching between the Loxone App and all sorts of other apps on your device. It can not only just open apps but also playlists, songs or saved maps. To do this you define a URL for the app to open. To add App into your program simply hit F5 and search for App. You can then define the URL, name and symbol.
You can find information on which URLs to specify either directly from the website of the app in question or on various sites on the Internet. Here are two example websites that are very useful when using App.
Below is an example of the information you would find on Google Maps with handleOpenURL: