What does the warning about mobile data charges mean that is displayed in the tolino app when I play/download an audiobook?

When you listen to an audiobook for the first time and have only downloaded a single chapter of the audiobook, the tolino app attempts to download the remaining data. This message allows you to decide whether you would like to download the data now or would prefer to wait until you are connected to a Wi-Fi network. You can download the audiobook using your mobile data if you are on the go or on holiday. To do so simply tap “Continue.”