The concept of torque is important in understanding how force is produced in weight training exercises. Its definition is simple: it is the magnitude of twist around an axis of rotation. Thus torque (twist) is rotary (angular) movement in any plane about an axis. Torque is seen in almost all movements of the body as, for example, when you do single joint actions.
In isolated movements the axis of rotation is fixed so that the bony lever moves in an arc of a circle. For example, in knee joint extension the foot circumscribes an arc of a circle because it is moving on an angular pathway. When you twist the shoulders they rotate around a stationary vertical axis (for example, the spine), and you make an arc of a circle when viewed from above.
When torque is produced, the force is applied at some distance away from the axis of rotation. For example, picture driving a car and turning the steering wheel. The hand applies a force on the wheel (rim) with the axis in the center of the steering column. This is known as an off-center force or, more accurately, an eccentric force. But the rotating or turning force is called torque.
It should also be noted that the axis of rotation can also be in motion. This is sometimes needed for safety. For example, in the seated leg extension exercise in which your thigh is immobile, the forces generated in the knee joint are extremely high when you straighten the leg. Because of this, this exercise has been negatively criticized as being potentially dangerous.