What is the parallel axis theorem?

The parallel axis theorem states that the "moment of inertia of any object about an axis through its center of mass is the minimum moment of inertia for an axis in that direction in space," according to HyperPhysics. It is also know as Steiner's theorem.

It can be used as a shortcut to find moments of inertia for objects when the axis of rotation is somewhere other than the center of mass of the object. If the inertia is known for objects when going through the center of mass, the new value of I can be found for any axis that is parallel to the center of mass axis and displaced by some distance from the center of mass, d.