Question

what do you mean by acceleration due to gravity

Answer 1

The acceleration which is gained by an object because of the gravitational force is called
its acceleration due to gravity.

Its SI unit is m/s2 . 

Answer 2

when any object dropped from a height to earth. earth exert a gravitational force on it and object accelerate on falling vertically downward..
g is used for acceleration due to gravity.
g= 9.8m/s^2