To listen offline, you'll need to do the following:
1. Add a title to your Library, by tapping on the library icon next to it.
2. Navigate to the Library view.
3. Tap on the 3 dots next to the title you want to download.
4. Tap on 'Download audio'.
Please note: Audio titles are only available for download in the apps for mobile (smartphones and tablets) and not on the website.