Couples and their Properties:

A couple is a force system contains two equal, coplanar, parallel forces working in opposite directions. As a couple constitutes two equal and parallel forces, their resultant is zero and therefore a couple has no tendency to produce translatory motion but generated rotation in the body on which it acts.

It shows two equal and opposite forces, each equal to P and working at A and B along with parallel lines, therefore constituting a couple. The perpendicular distance AB is known as the arm of the couple and is mention by p.