It's used to find the gravitational force between two objects with mass.
The equation is:
F = G * (m1 * m2) / r^2
m1 and m2 are simply the masses of the two bodies, and r is the distance between their center of masses.
G is defined as:
6.67300x10^(-11) with units of: m^3 kg^(-1) s^(-2)
So finding the force should be pretty straight forward.
Good Luck.