Some war machines, spells or magic weapons are so destructive that if a model suffers an unsaved wound from them, the victim doesn't lose only 1 Wound, but 2 Wounds, or a number of Wounds equal to the roll of a D3, D6, etc, as noted in the weapon's rules. In such cases, roll to hit and to wound as normal and then take any armour saves and ward saves that apply. Finally, for each such wound that is not saved, roll the appropriate dice to determine how many wounds are caused.
A model cannot suffer more wounds than it has on its profile. Should the model do so, it dies instantly and any excess wounds are wasted. You'll notice a single hit causing many wounds works differently to multiple hits that each cause a single wound. This is quite deliberate – it's easy to imagine a volley of arrows falling amongst the members of a unit and killing several, but a cannonball that hits a single Elf should always only kill a single Elf, regardless of how mighty the blow.