The concept of torque for an engine is the simplification of a basic principle of physics corresponding to the conjunction of 2 forces due to a point of support. The classic example illustrating the idea is the lever arm. In an engine, it is the piston-con-crankshaft assembly which, relying on the bearings of the block, thus creates a rotating movement. So it's the strength of the engine. Horsepower is the product of torque and engine speed. As a result, a large torquey engine pushes at low speeds while a motorcycle engine will need to take turns to express itself, for lack of torque.