Rotational inertia, or more scientifically known as moment of inertia, is a property of an object that determines the difficulty of accelerating the motion of the said object around a rotational axis. Because golf involves swinging the club in a circular or rotational direction, moment of inertia will always come into play.
Moment of inertia is affected by an object's shape and the position of the rotational axis around it. When a body has a large rotational inertia, greater force is needed to swing the object about the axis of rotation. Imagine a swinging pendulum and how it exerts an effort against the rod or string holding the weight. The farther the weighted object is from the axis of rotation, the greater force is that is tugging the string. The higher tugging force allows the string to resist unwanted rotations or movements.
The science in pendulums is similar to how a golfer swings his club. Golf clubs are designed to include the factor of rotational inertia, so the location of the weight on the club's head is of monumental importance to how the club will hit the golf ball. In the case of golf drivers, a player moving the center of gravity to his heel and toe will result in a slightly higher rotational inertia, and therefore a more precise swing and hit.