Helpless


 * "Paralyzed, held, bound, sleeping, unconscious, or otherwise completely at an opponent’s mercy. A helpless target is treated as having a Dexterity of 0 (-5 modifier). Melee attacks against a helpless target are automatic critical hits. You can always hit a helpless creature unless you roll a 1 on your attack roll." —from the in-game glossary

A PC or NPC is helpless when it is asleep, held, mesmerized, or has any of its Abilities reduced to zero. No action can be taken, objects cannot be interacted with (not even rest shrines or chests), and inventory items cannot be moved, traded, equipped or consumed.

Often this is caused by a spell such as Hold Person or Sleep.