The Apple AirPort Carrier Card Adapter (M8753G/A, 820-1066-A) is for the slot-loading G3 iMac (400MHz or faster). The “carrier card” allows a standard original Apple AirPort wireless card to be installed in an iMac.
At the time, wireless capability was not considered a standard computer feature so the iMac G3 not only did not include an AirPort Card, but it also lacked the internal port to accept it. Instead, Apple required this carrier to allow an AirPort Card to be added. The 802.11b AirPort Card was not included.