Pierce is an attack type and form of counter armor that typically ranged units will deal or specific siege units like bolt shooters, almost exclusively only ranged units will deal this attack type with some exceptions for specific hero units. Buildings and especially siege units like Siege Rams are very resistant to pierce damage.
Recent changes (alpha 15), have reworked the damage/armor system of 0 AD - resulting in Infantry Spearmen having all their dual pierce/hack damage converted into pure hack damage and similarly with cavalry units. Pierce Damage is now dealt almost exclusively by ranged units, and specific siege units.
Hellenes Spearmen typically have high pierce resistance and armor.