Strictly speaking, the description of Coulomb's law given on the previous page (written below this paragraph) is meant to apply to point charges. However, just as Newton was able to develop the mathematics(calculus) that proved the mass of any spherical object can be considered to be concentrated at a point at the centre of the sphere for all locations outside of the sphere, so it might also be proven that if charge is uniformly distributed over the surface of a sphere, then the value of the charge can be considered to be acting at the centre for all locations outside the sphere.
I got confused with the bolded portion. Would anyone mind elaborating on what they are trying to say here?
The description of Coulomb's Law (from the textbook): The electrostatic force between two point charges, q1 and q2, distance r apart, is directly proportional to the magnitudes of the charges and inversely proportional to the square of the distance between their centres.