As the name suggests, an armoured cable is protected against mechanical damage, whereas an unarmoured cable is not. Armoured cables are normally underground cables, which can be subject to damage from pieces of rock, etc., which may be in the trench backfill, or from excavation tools.
The 'armour' is a layer of overlapping metal (usually steel) tapes or wires that form the outermost metallic layer of a cable in order to protect the cable's waterproof sheath. This armour layer is itself protected against corrosion with a protective sheath