Whenever you strike someone in combat, your attack has to go through all their layers of armour before it can inflict damage (see armour). One way to get around this is with something called 'bypass'.
Certain talents, such as Cleave, Smash and Pierce, provide a bypass bonus to specific types of damage. In addition, some weapons have this as an innate ability for certain commands, which are coloured cyan (along with their AP cost) at the top of the combat table. It's also possible to receive bypass bonuses from certain powers or magical item bonuses - and these will all add together. To view your total bypass percentage, type 'armour bypass'.
Whenever you strike an opponent, every point of bypass will negate one point of your opponent's armour. If you completely negate their first layer of armour, you'll then move on to the second layer, then the third, and so on, until you run out of bypass or they run out of armour. For example, if you had 50 bypass and your opponent was wearing a 40% armour helm and a 50% armour coif, the helm would end up soaking 0% damage while the coif would soak 40% damage.