this is the whole description from the book:
"With use of coulomb's law for the force as a function of distance, as represented in this figure, we can calculate the amount of work that must be done to bring the two unit charges from an infinite distance apart to the distance 'r' apart."
the figure is a diagram illustrating the inversely square law of electrostatic repulsion: force is inversely proportional to the square of the distance between the charges. The numbers correspond to two 1 stoney charges.