Hitroll is a statistic that represents how easily you can penetrate an enemies defenses and land a blow.
The simplest description of how hitroll and armor work is that each hitroll cancels out approximately 6 armor on your target. So 15 hitroll is as effective against 90 ac as 25 hitroll would be against 150 ac. There's also a magic constant of 16 which makes it so that 0 hitroll is effective against the default 0 armor. Calculating this gives your base effectiveness against the armor of the target:
base effectiveness = hitroll - (target armor / 6) + 16
For example, 0 hitroll against 0 armor has a base effectiveness of 0, and 10 hitroll against 100 armor has a base effectiveness of -6. Note that negative numbers are bad and positive numbers are good.
This isn't the real effectiveness, however. Because some attacks hit harder than others, and sometimes you get lucky (or unlucky), there is a small random bonus added in. There is also a small blindness bonus but it is not known.
real effectiveness = base effectiveness + random(1 to 25)
This real effectiveness value determines how much of your damage is absorbed by the armor for each hit:
effectiveness less than 2 = no damage to target effectiveness 2 to 8 = 1/4 damage to target effectiveness 15 to 21 = 3/4 damage to target effectiveness greater than 21 = full damage to target
Also, regardless of the real effectiveness, any attack may do 0 to 9% damage, despite any armor.