Your AirPods case must be a wireless charging case (Qi) to charge wirelessly. Some AirPods, such as second-generation AirPods, came with either a standard case or a Wireless Charging Case.

To tell them apart, look at the status light: on a wireless charging case the LED is on the front outside of the case, while on a standard case the LED is inside, visible only when you open the lid. A standard case does not support wireless charging and must be charged with a cable.