Volumetric efficiency compares the volume of air entering a cylinder during intake, to the internal volume of the cylinder when the piston is at bottom dead centre. It is usually expressed as a percentage.
In a naturally aspirated engine, one without forced induction, volumetric efficiency can never be 100%. This is because of the resistance to the airflow that occurs at the carburetor, manifold, and the intake valve.
With forced induction, the incoming air is compressed, so that a greater volume and mass of air is forced into the cylinder during the intake stroke. That takes the volumetric efficiency to above 100%
