If something — especially a vehicle — is armour-plated, it has a hard metal skin on the outside to stop bullets, blasts, or heavy hits. Think of a money van, a soldier's car, or some police vehicles: extra metal plates fixed on. The word is mainly for machines, not for a person wearing a suit of armour. Writers sometimes stretch it: an “armour-plated” excuse or contract is one that is very hard to attack. That figurative use is a bit journalistic; the plain meaning is physical metal on a vehicle. The spelling armour is British; American English writes armor-plated.