If you have an iPhone with USB-C, you can use the phone itself as a small power bank for your AirPods when you are away from a plug.
Apple supports charging small accessories from iPhone 15 and later through the USB-C port at up to 4.5W. That includes AirPods, Apple Watch and other low-power USB Power Delivery accessories.
For AirPods with a USB-C charging case, connect the case directly to your iPhone with a USB-C cable. For older AirPods cases with Lightning, use a USB-C to Lightning cable instead.
The feature is particularly useful when travelling. If your AirPods case is nearly empty but your iPhone still has plenty of battery, a short top-up can be enough to get another listening session without finding a socket.
It is not intended as a replacement for a normal charger, and charging another device will of course reduce your iPhone’s own battery level. But in an airport, train or hotel room where you only have one charger available, it can be surprisingly handy.
You can do the same thing with an Apple Watch by connecting its magnetic USB-C charging cable directly to the iPhone.
There is no separate setting to enable. Connect the accessory and the iPhone handles power automatically.