I could explain it, but there is a
lot of information to cover.
I'd suggest you read these two pages first:
-
Armor calculation
-
Damage calculation
Those are very comprehensive pages, covering how armor and damage works in Guild Wars.
If you have any questions after reading those, please ask.
Edit: if you just want to know how armor translates into a damage multiplier (e.g. 100 armor is 50% damage): see the tables on the
Armor rating page. Keep in mind that armor boosts only work against armor-respecting damage.
In short, +10 armor is -16% damage. So if you are taking 200 damage without the buff, you will take 168 damage with the buff, a reduction of 32 damage. That doesn't even equal -33 damage, so only in extreme cases (210+ damage), will +10 armor reduce more than 33 damage.