Damage Reduction (DR): A special defense that allows a creature to ignore a set amount of damage from most weapons, unarmed attacks, or natural weapons, but not from energy attacks, spells, spell-like abilities, and supernatural abilities. The number in a creature’s damage reduction is the amount of hit points of damage the creature ignores. The information after the slash indicates the type of weapon (such as magic, silver, or evil) that overcomes the damage reduction. Some damage reduction, such as that of a barbarian, is not overcome by any type of weapon.
Barbarian Class Ability
Damage Reduction (Ex): At 7th level, a barbarian gains Damage Reduction. Subtract 1 from the damage the barbarian takes each time he is dealt damage from a weapon or a natural attack. At 10th level, and every three barbarian levels thereafter (13th, 16th, and 19th level), this damage reduction rises by 1 point. Damage reduction can reduce damage to 0 but not below 0.
Source: Player's Handbook