Keep in mind that NWN2 uses the 3.5 take on Damage Reduction.

In previous editions of AD&D, you had the black and white "must have +X weapon or better to hit" system: a weapon that did not meet muster could not damage the enemy at all. The BG engine treated it as if the weapon had not even connected, while the Gold Box games would consider it to be a 0 damage hit (which led to some interesting foibles, for instance if you made an enemy helpless - which meant any physical attack was a one-shot KO - you could KO them with weapons they should not normally be able to be damaged by ...).

In 3.0, which was used in NWN1 and in Icewind Dale 2, D&D transitioned to a system called "damage reduction" - combatants with DR have a certain fixed number of points knocked off the top of any physical hits taken. Sometimes a certain weapon plus would be able to overcome the damage reduction (e.g., "10/+2" meant hits from weapons not +2 or better did 10 points less of damage than otherwise, to a minimum of 0); sometimes DR could not be overridden at all (e.g., the Barbarian class feature, this was usually denoted as "3/-" or such).

In 3.5, this was tuned more finely; now damage reduction is usually overcome by specific categories of weapon rather than by a specific "plus" - this allows a split between weapons that are inherently powerful and ones that are more effective against certain types of enemies that the previous incarnations mainly didn't cater to as a baseline system feature. Now you have something like "5/Cold Iron" or "10/Adamantite" where any weapon that doesn't meet the category requirement is subject to the subtraction, no matter how powerful (in these cases, a non-enchanted cold iron or adamantite weapon will penetrate the resistance, but a +5 steel weapon will still be resisted). Sometimes magic weapons are the category (e.g., "5/Magic") but this is less common, and a specific plus beyond +1 is almost never the requirement.

On top of this it is also possible for combatants to have resistances to the physical damage categories, and these stack (e.g., something can have 10/Silver and blunt resistance 10, in which case a silver long sword will deal full damage, a steel dagger will lose 10 points of damage and a wooden quarterstaff, being both blunt and nonsilver, will have a whopping 20 taken off the top of every hit).

It all makes things much more strategic and requires you to often carry a wider arsenal than used to be the case (where you just needed usually the biggest plus weapon of whatever type you had put the most feat investment into).