To understand rotational inertia, we had better explain inertia in general.
Inertia is a resistance to change of motion.
The more inertia an object has, the less it responds to being pushed.
For linear motion (moving from here to there),
The measure of an object's inertia is its mass.
The more mass in an object, the less it responds to being pushed.
A light push sends a ping-pong ball flying, while a light push on a bowling ball has almost no noticeable effect.
Rotational inertia is a resistance to change of rotation.
The more rotational inertia an object has, the less it responds to being spun.
The outside parts of a spinning object have to move much faster than inside parts near the axis.
How an object is built, and along what direction you want to spin it, also affect the rotational inertia.
Another form of inertia is rotational inertia (→ moment of inertia), which refers to the fact that a rotating rigid body maintains its state of uniform rotational motion. Its angular momentum is unchanged, unless an external torque is applied;
this is also called conservation of angular momentum. Rotational inertia often has hidden practical consequences.