A vehicle with armour plates has extra protection against hits. Roll a D6 for each glancing or penetrating hit caused against it. On a 6 the hit is ignored because it's deflected by the armour. Armour plates will have no effect against ordnance hits 'cause they're just too big! There are two costs for armour plates. The cheaper one is for vehicles worth 50 points or less (including any other upgrades they have apart from the armour plates) the second is for vehicles that are worth 51 or more points.
Armour Plates
Source: Codex: Orks, p. 37