If during the confirmation of a critical hit, you roll a 20 on d20, then the damage multiplier on the attack is increased by +1. In addition you may roll a d20 an additional time, if on this 3rd roll you roll a 20 then the multiplier is increased by an addition +1, this process can continue until you no longer roll a 20 on a d20.
Example: Olav the Barbarian hacks at the Fiendish Bugbear with his Great Axe and rolls a 20 on his d20 Attack Roll. When rolling again to confirm his crit he rolls another 20. A Battleaxe's Crit mod is x3, but now with this last roll it is at x4. Olav rolls the d20 a second time, by the heavens he has rolled another 20. His Crit mod is now x5. Olav rolls again but alas he has only rolled a 5. However when Olav rolls for damage on his attack, he is able to multiply his base damage by x5, instead of x3 as he normally would.