What is meant by acceleration due to gravity?

Derive an expression for it in terms of mass of the earth and gravitation constant.

According to Newton’s law of gravitation, every mass exerts a gravitational force on every other mass in the Universe.
Thus, the earth exerts a force on every mass in its surroundings. This force of attraction exerted by the earth on a body is called the force of gravity and the acceleration possessed by a body moving under the force of gravity is called acceleration due to gravity. 
The value of acceleration due to gravity does not depend on the mass of a body but it depends on the mass of the earth and distance of the body from the center of the earth.
Consider, the earth to be a uniform sphere of mass M and radius R.
Let a body be placed at a distance r from the surface of the earth.
The gravitational on the body is, 
                 
The acceleration produced in the body due to force of gravity i.e. acceleration due to gravity is, 
                 
At the surface of the earth, the acceleration due to gravity is,