The acceleration due to gravity, usually written as g, is a measure of how fast a free-falling object will accelerate when dropped near the surface of the Earth. It is more or less constant everywhere on Earth.
The acceleration due to gravity on Earth comes from from Earth's large mass and the fundamental force of gravity, which is one of the four fundamental forces and is noticeable near large astronomical objects (like planets, moons, and Suns). There are minor variations caused by differences in density on the Earth. The weight of an object depends on this acceleration due to gravity and its mass, represented by the formula:
To see how the acceleration due to gravity changes above the surface of the Earth please see hyperphysics.