The amount of Armor a hero can ignore while attacking with physical damage in an ability(if it is physical) or an autoattack. Also known as 'penetration' or 'armor ignore', this stat is usually attributed to usually to STR or AGI based Heroes as most, or a large part, of their damage is physical.
Many heroes can provide this as a Debuff known as Armor Reduction/Redux on the enemy team shown by the symbol of a broken shield: