From The Heroes of Might and Magic III wiki

Reduces the damage done to your hero's creatures. Counts after the other damage modifiers. Does not affect magical damage

Recommended for: Any Hero who can afford the slot.

Level	Damage reduced by
None	0%
Basic	5%
Advanced	10%
Expert	15%

Heroes with a specialty in this skill receive a 5% per level bonus:

• Mephala a Ranger
• Neela an Alchemist
• Tazar a Beastmaster