Armor can be bought from the Armorers and equipped to a Mech that meets the needed requirements from Your Armory. Armor reduces damage from certain damage types by a certain percentage, and this percentage decreases as a Mech is hit and is damaged in a battle, due to armor erosion.
Each piece of armor has two requirements that must be met before they can be equipped: a level requirement, and a Movement requirement. The weight of the armor will be deducted as a penalty to the mech's effective Movement, slowing it down. Armor can be modified at the Plasma Smith to upgrade one of its defense types, or to give special effects such as resistance if you have the necessary commander skills.
As a Mech is hit, it's armor and defence both degrade. The stronger the hits, the more the defence/armor degrades.
Try to avoid a Mech being targetted by the entire enemy team, as it will result in severe armor and defence damage.
Armor and Defence resets once the battle is over.
With 10% armor and 10% defence you block:
10% from the damage you take, and then another 10% from the remainder.
So if the damage was 100,
100-10% = 90
90-10%= 81 damage (not 80).
Resistance reduces the effect that negative effects and criticals will have on you. For example, if you are hit with a negative effect and have 10% resistance, you will only recieve 90% of the negative effect. Negative effects include Slowing, Jamming, Critical damage and armor and defense degradation.