Fallback is a special app, only by command line interface, which provide a way to have a secondary server which you can used if your main server goes down. This other server will allow you to deploy a copy of your server to bring back you to internet during your break down.
After the installation, you should not have anything else to configure. If you want anyway, you can find the list of app to backup in the file
/home/yunohost.app/fallback/app_list and a global configuration in this other file
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