Octane,
Negative torque is when the prop is driving the engine. Normally, the torque is positive and the engine drives the prop. When an aircraft engine is being driven by the prop, it means that the powerplant is extracting considerable energy from the air stream - which in other words means "creating considerable drag".
There are lots of different terms that all mean the same thing:
- "Windmilling"
- "Backdriving"
- "Discing"
- "Negative torque"
You would be familiar with negative torque in your car. Take your foot off the accelerator and allow the car to coast. The revs will stay high, but this is because the wheels are now driving the engine's rotation. You have switched from positive torque to negative.