that means the port, the plastic tube that makes the port has a volume that needs to be acounted for. they are saying that their measurment has taken this into acount, kinda like the volume of the sub that is inside of the box
I second what someone else said use a sealed box. yes you need more power to drive it but it is much cleaner sounding then a ported box and it will respond better, a ported box will just be loud
