“You've become an expert with the Auto Axe. You do +25% damage whenever you use it. Vroom.”
Auto Axpert is a perk added to Fallout 3 by the The Pitt add-on.
As the description states, this perk boosts the Lone Wanderer's damage by 25% when using the auto axe or one of its variants (steel saw, The Mauler, Man Opener).