In order to fly level at a constant speed, the total drag acting rearwards must be balanced by an equal and opposite force acting forwards: thrust.
When thrust is greater than drag the aircraft will accelerate, and when thrust is less than drag the aircraft will decelerate.
In small aircraft, the thrust is created by the propeller which pushes a large mass of air backwards.
If the engine fails in flight, the propeller will often continue to rotate as a result of the airflow over it (similar to a windmill spinning in the wind).
The propeller is connected to the crankshaft and in turn connected to many heavy metal parts in the engine, all of which will continue to rotate if the propeller is windmilling. The energy needed to rotate all this weight has to come from somewhere and so it is experienced as drag on the aircraft.