What is meant by a twisting couple?

Hint: Let us know about the couple. A pair is a system of forces having a consequent (also known as net or sum) moment but no resultant force in mechanics.
Force pair or pure instant is a better term. It has the effect of causing rotation without translation, or more broadly, without acceleration of the centre of mass. Force couples are free vectors in rigid body mechanics, which means their effects on a body are independent of the point of application.

Complete step-by-step solution:
A torque is the resulting moment of a pair. This is not to be confused with the term torque, which is essentially a synonym for moment in physics. Torque, on the other hand, is a type of moment.
When the plane of the couple is parallel to the plane of the cross-sectional of the member, it is called a twisting couple.
Torsion is the twisting of an item caused by an applied torque in solid mechanics. Torsion is measured in newton metres (Nm) or foot-pound force (ftlbf) and is expressed in Pascal (Pa), a SI unit for newtons per square metre, or in pounds per square inch (psi).

Note: In mechanical engineering and the physical sciences, couples are extremely essential. Here are a few examples:
The pressures exerted on a screwdriver by one's hand
The forces produced on the head of a screw by the tip of a screwdriver
A spinning propeller is subjected to drag forces.
In a homogeneous electric field, the forces acting on an electric dipole.
A spacecraft's response control system.