It just means that if you have a weapon with, for example, 36" range, when firing at a vehicle with Night Shields the weapon has a maximum range of 30". If you are more than 30" from the firer you could not be hit,
A meltagun (12") would only have a 6" maximum range and would only get the extra armour pen dice within 3".
A bolter would have a maximum range of 18" and rapid fire range of 9" and so on.