Complex sentence: This means the total entropy of the net system (the entropies of the hot furnace, the "working fluid" of the Heat engine, and the cold sink) remains constant when the "working fluid" completes one cycle and returns to its original state.
Split into simpler sentences: This means the total entropy of the total system consisting of the three parts: the entropy of the hot furnace, the entropy of the "working fluid" of the Heat engine, and the entropy of the cold sink. The total entropy of the system remains constant when the "working fluid" completes one cycle and returns to its original state.