In your hero's profile you can see your resistance points (you get them with equipment).
The second number (in brackets) shows you how much of them you have due to your guild-skill "Improve armour".
The effect of your armour is to lessen your opponents combat damage. If somebode has an attack range from 168-220 damage and you have high armour you wont get the maximal damage - perhaps something under 200 damage.
Here a liddle fighting example: (will be edited)