FOD (foreign object detection) is a security guarantee that must be included in Qi certification.
This means that with a Qi-certified product, the transmitter of the wireless charging system can detect if a foreign object, such as metal, is on its surface. In this event, the wireless charger either stops charging or does not start charging at all.
If the wireless charger does not have a FOD, it will continue to heat up causing the object placed near the wireless charger or between the charger and the phone (such as the phone case) to become very hot. This can lead to very serious injuries.
Therefore, the FOD function is not only necessary for Qi certification but also provides a better guarantee for the user's use of safety.