Damage Resistance is damage reduction against a specific form of damage (see Damage type), but is not overcome under any circumstances. It is always written as type x/- where x is the amount of damage of that type to ignore. The dash indicates that there is no way to bypass the reduction.
As of NWN2 v1.23, Damage Resistance does not work as it should; it works properly for Energy and Elemental damage (fire, cold, sonic, negative, etc.), but not for physical forms of damage (slashing, bludgeoning, & piercing). For example, a Swordsman's Belt states "Resistance: Slashing 5/-" but it will not subtract any damage from slashing blows (or any other damage) against the wearer. However, note that you will get a message to the effect, "Damage Resistance absorbs 5 damage" ( ! ). The only flavor of physical Damage Resistance that seems to work is the 'untyped' kind:
- ... actually, there's one type of physical Damage Resistance that does work, and it's the untyped kind, as seen on Adamantine armor, and the Ironskin Chant, and Dwarven Defenders, and etc. (although the resistance on Adamantine shields is bugged as well, and does nothing) - filaminstrel
The link is to an interesting and informative thread on the old Bioware board. Best read both pages.
Also note that the Hardness resistance of doors, chests, and other objects does not subtract damage, physical or any other.