Hardness is Damage reduction for objects.
Hit an object the first HARDNESS points of damage are ignored, the rest damage its Hit Points.
once the HP reach zero the object is destroyed e.g. hole in the wall, sword smashed, paper chopped into pieces.
e.g.
An object has 10 hardness and 5 HP
it gets hit for 8 damage, it takes NO damage at all, as the hardness takes it all.
It gets hit for 11 damage.
10 is ignored, it takes one hp
It then gets hit for 18 damage.
Hardness ignores 10 and the remaining 8 is applied to the object bringing it up to 9 HP damage. More than it has, the object is destroyed.