Quick questions on Newton's law of universal gravitation explained: HSC Physics Module 5

Force is inversely proportional to the square of the distance. Doubling $r$ reduces the force to one quarter. Halving $r$ quadruples the force. This rapid fall-off explains why Earth's gravity dominates near the surface but becomes negligible far from the planet.

The gravitational field strength $g$ at a point is the gravitational force per unit mass on a test mass placed there:

A gravitational field is a region of space in which a mass experiences a force. Around a spherical body the field is radial: the field vectors point toward the centre of the mass from every direction, and they get shorter (weaker) with distance, since $g \propto 1/r^2$.

For an object of mass $m$ in free fall in a gravitational field $g$, the acceleration is $a = g$ (regardless of $m$, because $F = mg$ and $a = F/m$). All objects fall with the same acceleration in a given gravitational field, in the absence of air resistance.