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