Just like in the real world, fire is the combustion and burning of a fuel source that results in massive amounts of heat.
In the fantasy world of Crossfire, it is has weaponized. And the fuel source just may be magical or from supernatural origins.
The fire attack can come from multiple sources - melee and range weapons, spells, monster, players (i.e., flame touch), ground tiles (i.e., lava) and traps.
It is possible for the fire attack to destroy items such as magical and non-magical scrolls and books. And given a long enough exposure times, it can also destroy metal armour and weapons. And, just about everything else in between.
In some cases, a player can outrun the area of effect from a raging fire from certain monster attacks, spells or trap effects. But in cases where the fire attack is more direct - items, potions and natural resistance to fire is necessary.