Data roaming on an iPhone allows you to use your mobile data connection when you are outside of your carrier's network coverage area. This typically occurs when you are traveling internationally or in areas where your carrier does not have a strong signal. However, using data roaming can incur significant charges, so it's important to understand how to manage your data roaming settings on your iPhone to avoid unexpected fees. You can typically find these settings under Cellular > Data Roaming in your iPhone's settings menu.