You make a single attack that deals significantly more damage than normal.
When you use the attack action, you can make one attack at your highest base attack bonus that deals additional damage. Roll the weapon’s damage dice for the attack twice and add the results together before adding bonuses from Strength, weapon abilities (such as flaming), precision based damage, and other damage bonuses. These extra weapon damage dice are not multiplied on a critical hit, but are added to the total.
The text on this page is Open Game Content, and is licensed for public use under the terms of the Open Game License v1.0a.
Sources:
SECTION 15