Flammability is a Stat: How easily an object catches fire and how strongly it burns.
Note that having a flammability of 0% only prevents the item/pawn/etc catching on fire, it does not prevent taking Flame damage. For example, while a stone wall will not be ignited by molotov cocktails, and won't continue to burn, it will take the 10 flame damage from the initial attack. Similarly, a pawn requires Armor - Heat to actually mitigate flame damage taken. Some things, such as the buildings found in mech clusters , have both a flammability of 0% and are immune to flame damage but these are separate effects.