Air handler referred to in your answers. Please tell me what that is.

Your heating and air conditioning systems come in different parts. If you have a heat pump or AC, you have an outside unit. Inside there is the heater burner assembly, the evaporator (if you have AC) and the fan unit. That is what is referred to in the trade as the air handler. It contains a cabinet, a motor and a caged fan attached to the drive motor. It's only function is to move air, therefore "air handler".