Smite evil

Once per rest period, a Paladin may attempt to Smite evil with one normal melee attack. The number of times a Paladin can Smite Evil increases by 1 at level 5 and 10 and can be further increased by taking the Extra Smite Evil Enhancement.

She adds her Charisma modifier x 2 to her attack roll (that uses the best attack bonus in a normal attack sequence) and deals extra damage based on her paladin level. A Paladin with 12 Charisma (+1 modifier) and BAB 0 would add 2 (1 x 2 + 0) to the attack roll. A Paladin with 20 CHA (+5 modifier) and BAB 10 would add 30 (5 x 2 + 20) to the attack roll (because the 4th attack is at BAB+10 = 20).

The extra damage done is  If the paladin accidentally smites a creature that is not evil, or is not in range, the smite has no effect, but the ability is still used up.