The universal law of gravitation describes the force of gravity between two bodies. What does it say about the strength of the gravitational field? How does the size of the object affect the use of the gravitational field equation?

i know that the gravitational field strength is
a=Gm/r^2
but what does the last sentence mean and what is it telling me to do ???

This law describes the interaction of the POINT masses (A point particle is an appropriate representation of any object whose size, shape, and structure is irrelevant in a given context)