The Hard\Soft rule is an RPG cliche listed on several internet lists of such things, and demonstrated in a significant amount of RPGs that mentioning it as a cliche is hardly clever.
Basically stated, this cliche means that heroes, heroines and sometimes antagonists are able to withstand massive amounts of damage, such as a blast of dragon fire or a direct hit with a meteor while in the middle of a random combat; but the same character can be killed simply and easily by a sword, dagger or bare hands when a FMV comes on for plot purposes.
This is related to such rules as the fact that even characters who have miraculous resurrection powers can't save their friends who have died from plot twists, and the fact that characters with enough firepower to kill the most powerful bosses can't do anything about a drone jam who is standing in the way of an important door.
This cliche was probably demonstrated most famously in the case of Aeris Gainsborough, although her story could be said to transcend the province of Console RPG Cliches.