From The Heroes of Might and Magic III wiki
Armorer is a secondary skill, that reduces the physical damage done to hero's creatures. Physical damage means damage done by enemy creatures engaged in melee or ranged combat. Armorer secondary skill does not reduce damage from spells cast by enemy heroes or creatures.
Heroes who specialize in armorer skill:
It should be noted, that due to a bug in the game, the damage from arrow towers during siege combat is not reduced by armorer secondary skill, but actually increased the corresponding amount.
Armorer skill is unquestionably one of the most useful secondary skills in the game.
At expert level it reduces 15% of all physical damage, which, though not strictly speaking true, effectively increases allied creatures' health by 15%.
This on the other hand leads to less casualties in battles, and more troops for the next battle. Especially in extra large maps this may give the needed edge to vanquish the enemy and bring peace to people that did not necessairily expect it in given quantities but were quite horribly anguished at not supplying any.
 Creatures with Similiar Features
Golems, which can reduce induced magical damage (Tower units Stone Golem have 50% magical damage reduction, Iron Golems have 75% damage reduced and the neutral units Gold Golem have 85% magical damage reduction and finally 95% magical damage reduction Diamond Golem)
 Creatures with Healing (Regenerating or Life Drain)
 Spells with physical damage reduction
- Air Shield, 3rd level Air Magic spell, reduces physical damage from ranged attacks
- Shield, 1st level Earth Magic spell, reduces physical damage from melee attacks