The COM map is the mechanism that exposes interfaces on an object to a client through QueryInterface.
BEGIN_COM_MAP( x )
[in] The name of the class object you are exposing interfaces on.
CComObjectRootEx::InternalQueryInterface only returns pointers for interfaces in the COM map. Start your interface map with the BEGIN_COM_MAP macro, add entries for each of your interfaces with the COM_INTERFACE_ENTRY macro or one of its variants, and complete the map with the END_COM_MAP macro.
From the ATL BEEPER sample: