Question

What is moment of inertia and explain it ?

Answer 1

a quantity expressing a body's tendency to resist angular acceleration, which is the sum of the products of the mass of each particle in the body with the square of its distance from the axis of rotation.

Answer 2

Inertia is the resistance of the object to any change in its motion, including a change indirection. 
An object will stay still or keep moving at the same speed and in a straight line, unless it is acted upon by an external unbalanced force.