The Offline Mode is not really a feature, but it allows your users to download the contents of the app in their phones so that they can access it later, even if they have no connection. Let’s see how this works.
When the users download the app and open it for the first time, they will be asked if they want to download the contents to access it when they are offline. If they do it, they will be able to access all the contents that are in the app even when there is no connection.
But you have to know that everything that is linked to an external website will not be downloaded, like for example Youtube videos, Instagram galleries, or Facebook pages. As you have to get the content from outside the app, you need an internet connection to access it.
Note: If you have published an app before the version 3.9, you have to republish it after you have updated to the version 3.9 (or higher) so that the offline mode is enabled for your app.