I am unable to get it. . .can i have full explanation (informal way ) plzz ? Would you be kind enough to help me ?

The theorem of parallel axes states that the moment of inertia of a body about any
axis is equal to the sum of the moment of inertia of the body about a parallel axis
passing through its centre of mass and the product of its mass and the square of the
distance between the two parallel axes.

Suppose a rigid body is made up of n particles, having masses m1, m2, m3, ... , mn, at
perpendicular distances r1, r2, r3, ... , rn respectively from the centre of mass O of the
rigid body.
The moment of inertia about axis RS passing through the point O:

 It is very simple..

1. It is used to find the moment of inertia of the axis parallel to that of centre of mass.

2. Its general formula is ( I_AB = I_cm + Md² ) .... where AB is the parallel line , and CM is the line passing through Centre Of Mass , d is the distance between the two axis.

This is all parallel axis theorem..