Armour is an attribute of certain units that affects damage.
A unit is either armoured or unarmoured.
Armour affects how much damage a unit recieves from certain attacks. Some attacks ignore armour (giant's attacks), some have their damage reduced by armour (Archidonis Arrows), and some even deal extra damage to units with armour. Albowtrosses with blazing bolts do more damage to armoured units than other units.
Units with armour play a sound similar to a metallic ting when hit by an attack. Unarmoured units play the sound of flesh being hit.
Attacks that ignore armour: All Spells, Poison, Giant Attacks (both), Shinobi
Attacks that do increased damage to armour: Albowtross with Blazing Bolts, Fire Arrows (Since Patch 1.88)