Exactly two things:
1). The product of the two masses.
2). The distance between the centers of the two objects.
Comments:
-- It doesn't matter whether the objects have nearly the same mass, or whether
one is huge and the other one is tiny. It's their product that counts.
The Earth pulls you toward it with the same force as you pull the Earth toward you.
The Earth's weight on you is the same as your weight on the Earth.
-- The force of gravity between two objects doesn't depend on anything between
them. There's no way to block gravity or shield anything from it.
