Ability score damage is an effect which temporarily reduces one of more of a creature's Ability Scores, such as an injury. This reduction does not affect the creature's base ability scores, but applies a penalty to all checks related to that score. It usually occurs as the result of a serious injury, usually due to a critical hit or critical failure.
Ability score damage can lead to death if left untreated. Characters can use temporary ability score bonus effects to offset the effect of the injury, but require proper medical attention to clear the damage permanently. A character with any effective ability score at 3 or less is considered to be severely handicapped by the effect. Any ability score being reduced to zero causes the character to start dying.
3 or lower Strength: You're so weak that you may need help standing or walking, and can't lift any heavy items.
3 or lower Dexterity: You're unable to move without noticeable pain. Parts of your body may be completely paralyzed.
3 or lower Constitution: You're extremely frail and could be knocked out by even a minor blow.
3 or lower Intelligence: You're incapable of complete thoughts or sentences, and may experience anterograde amnesia (inability to form new memories.)
3 or lower Wisdom: You seem spaced out, unaware of what's going on around you. You may experience hallucinations.
3 or lower Charisma: You act as though you are unaligned - animalistic, paranoid, and detached from reality.