Answers above correct. The torque of the motor is working on a body of mass, the crankshaft, the conrods, the prop, the body of the plane wants to rotate in the other direction because of Newton's law (equal and opposite reaction).
With inline engines some of the torque is transmitted torsionally along a long crankshaft, so it spreads out over a greater area. This is why radial engines produce sharper torque rolls because the torque from the motor is being transmitted onto a small crankshaft and is felt more suddenly and dramatically by the airframe.