Power Attack
You suffer disadvantage on a melee or ranged attack roll to add an extra damage die to the roll.
A greataxe would increase damage from 1d12 to 2d12.
A greatsword would increase damage from 2d6 to 3d6 with a power attack.
With GWM or Sharpshooter, the bonus damage doubles, thus a great addition in power attack with GWM would deal 3d12 and a greatsword would deal 4d6.