Some creatures are naturally vulnerable to fire. Once flame has been set amongst such a beast's flesh, it will run rampant, causing terrible harm.
If a model with the Flammable rule suffers one or more unsaved wounds from a Flaming Attack, each unsaved wound is doubled. So, for example, if an attack would normally cause 1 wound, the Flammable creature would take 2 wounds. If the number of wounds are randomly determined by a dice roll, double the result of the dice, rather than rolling two dice and adding the scores together.