Guaranteed damage reduction
Wearing any body armour with AC greater than 2 provides you with an amount of guaranteed damage reduction (GDR) in melee combat. Normally, each attack that hits you is reduced by a random amount up to your AC. GDR is a form of insurance against poor AC rolls; if you have sufficient AC, you are guaranteed to reduce the damage taken by a percentage of the enemy's maximum attack damage equal to your GDR up to half of your AC. As an enemy's damage output is about as variable as your AC, a high GDR will allow you to completely negate an enemy's attacks much more often than normal, and those attacks that do get through will always be significantly diminished.
In most cases, your GDR% = (14*(Body Armour Base AC-2)^(1/2))%. No other type of armour provides any benefits to GDR.
GDR of Various Armours
|Robe, animal skin||2||0%|
|Troll leather armour||4||19%|
|Ring mail, steam dragon armour||5||24%|
|Scale mail, mottled dragon armour||6||28%|
|Swamp dragon armour||7||31%|
|Chain mail, fire dragon armour||8||34%|
|Ice dragon armour, quicksilver dragon armour||9||37%|
|Plate armour, pearl dragon armour, storm dragon armour, shadow dragon armour||10||39%|
|Gold dragon armour||12||44%|
|Crystal plate armour||14||48%|
There are also two Transmutations spells which allow you to have GDR in spite of the fact that they effectively disable your body armour: Dragon Form (34%) and Statue Form (39%). Finally, gargoyle characters receive an innate boost to GDR. This boost is inversely proportional to the GDR provided by your body armour.
What GDR Affects
GDR only works against one thing: physical damage dealt by monsters attacking you in melee (including via a reaching brand). It does not work against any sort of ranged attack, be it from a physical launcher or a Conjurations spell that deals physical damage. Neither does it work against elemental melee damage, such as an ice beast's cold damage. However, most such elemental attacks need to deal physical damage to you before their added effects trigger, so high GDR will make them happen less often.
Guaranteed Damage Reduction Cap
GDR is not a form of damage reduction independent from AC. If you have excellent GDR and terrible AC, it will do you little good. The damage reduction is capped at your GDR% of the enemy's maximum damage or 1/2 of your AC, whichever is lower.
For example, let's say you have 48% GDR and 100 AC, and you're facing an opponent capable of dealing up to 50 damage in a single hit. This would give you a guaranteed damage reduction of 24 (48% of 50), taking full advantage of the 48% GDR. The same 48% GDR coupled with 20 AC would only give a guaranteed reduction of 10, a mere 20% of the enemy's max damage, because it is capped at 1/2 of your AC. If you are instead facing an opponent capable of dealing 200 damage, then even at 100 AC, you'd only be guaranteed 50 damage reduction (25%), while the 20 AC character would still be stuck with 10 (which is now only 5%). In each of these cases, the GDR is still helping; even in the case of 20 AC vs. 200 max damage, it still mitigates the worst possible rolls.
This guaranteed damage reduction is effectively a minimum AC roll. If your damage reduction is 24 (as in the first example above) and you roll a 1 on your AC against a given attack, your roll will be boosted up to 24 instead. But if you roll a 36 on your AC, you won't get any bonus from GDR because you're already above the minimum.
GDR's usefulness depends heavily on how often you find yourself in melee combat, how high your AC is, and how much you're willing to sacrifice your evasion defense (in general, the worse an armour is for your evasion, the higher its GDR will be). Unless you intend to wade into close combat whenever possible, and you have no intention of training Dodging, you'll likely be better off avoiding super-heavy armour which maximizes GDR and focusing on optimizing your AC, EV, spells, and resistances to keep yourself alive. Characters that actively avoid melee, whether it be through fast movement speed or ranged attacks, will barely notice the effect of their GDR.