Moment of inertia:

The moment of inertia describes how strongly a body resists its rotation change.

The moment of inertia of a body depends on its shape, size, mass and additionally on its axis of rotation.

If the mass is moved closer to the axis of rotation, the moment of inertia decreases.

An example:

Just watch a figure skater who is doing a pirouette. If the figure skater pulls his arms towards his body, he will rotate faster. In doing so the moment of inertia of the arms relative to the axis of rotation is decreased.