G denotes acceleration due to gravity. example: An object is falling freely under action of only gravitational force ( means no air resistance, etc. ) it will have an acceleration equal to value of g. Value of on earth ranges from 9.82-9.87, and approximated to 9.8m/sec^2. Different acceleration due to gravity at different places is due change in height from center of earth or due to rotation of earth. Also, we can find approximate value of g on a planet by formula, g = GM/R^2 here, G is universal gravitational constant which has a value of 6.673 * 10^-11 Nm^2/kg^2 M is mass of that planet R is radius of that planet or distance between centre of that planet and that object. So, we can easily find out that g is independent of mass of object, density of object,etc. It means all object falling freely have same g or acceleration. This is enough for Class 9th but if you want to know more about it, I am giving some additional formulas and concepts. g'= g(R/R+h)^2 to find new g for change in height. g on moon is approximately 1/6th of on earth and approx value is 1.67m/sec^2. There are huge data on this, you can use 11th class NCERT to know more about it if you are interested enough.