Air will pass through porous material if pressure is applied. The resistivity of the material is the measure of this effect.
It is the ratio of pressure applied to the velocity of the air flowing through the porous materials.
This is similar to the basic electric circuit where a voltage is applied across a resistor and current flows.
A porous sheet of material acts like a set of resistors in parallel.
The more resistors in parallel, the more current flows and the lower is the overall effective resistance.
Less resistors yield higher effective resistance…