For functional, invisible armour?
It changes how damage is dealt.
There are 3 kinds of attacks in Stick Empires, armour-piercing, health-damaging and normal.
Armour piercing attacks are rare. I think the only armour piercing attack is the albowtross arrows. Maybe the magikill electric wall, but I don't think so.
Armour-piercing attacks do regular damage to non-armoured units, but do much more damage to armoured units.
Health-damaging attacks are probably the most common. They do normal damage to health, but less damage to armour.
Normal attacks do equal damage, regardless of unit. Normal attacks include shadowrath attacks, giant attacks, poison, bombers, magikill explosion, hell fists (maybe?)
The only armoured units though are speartons and juggerknights though.
I may be completely wrong on anything in this post though.....