Obstacle penalty is a term for reduction of damage for ranged attacks. It occurs only during sieges when the target is behind walls. The attack suffering from the obstacle penalty only does half the damage compared to an attack made against a target being in front of the wall. The obstacle penalty is cumulative with the range penalty.
Sharpshooters, Enchanters, Magi and Arch Magi ignore the obstacle penalty, as do all creatures under a command of a hero with either the Golden Bow or Bow of the Sharpshooter artifact equipped.