This is Newton's Universal Law of Gravitation. It states that the gravitational force exerted on one point particle by another is equal to the universal gravitational force constant G times the product of the masses of the two particles divided by the square of their separation.
Note: The symbol r represents separation here, not radius.
Pitfall Avoidance Note: In the case of objects rather than particles, the separation r is the center-to-center separation. One should be careful to avoid the use of the surface-to-surface or center-to-surface separation instead.