Using these rules weaponry is made more lethal. These rules allow even the most inexperienced fool with a weapon to potentially get a lucky strike in to deal more damage. It also strengthens the potency of weapons with large dice, but increases the chances of smaller dice landing more hits in.
Weapon damage dice explodes. Dice can only explode a number of times equal to the die used (d4 can only explode up to a maximum of 4 times). Weapons that use multiple damage dice still explode, however additional dice not from the weapon do not explode. An effect such as Rapid Strike count towards the amount of dice exploding, however effects such as additional fire damage doesn't explode.
These exploding dice affect weapons, not spells.