The moment of inertia, otherwise known as the angular mass or rotational inertia, of a rigid body is a tensor that determines the torque needed for a desired angular acceleration about a rotational axis. It depends on the body’s mass distribution and the axis chosen, with larger moments requiring more torque to change the body’s rotation. It is an extensive (additive) property: the moment of inertia of a rigid composite system is the sum of the moments of inertia of its component subsystems (all taken about the same axis).

A step by step explanation in this video:

Credits on this video: ECUSW