When attempting any d20 success roll, you have a 5% chance of rolling a “20”. This is called a Critical Success. You succeed in performing the desired action, AND get an awesome bonus effect.

A special case of the Critical Success is the Critical Hit, which occurs when you roll “20” on a Attack Roll. The player then gets to choose one of the following bonuses on their damage roll:

1. double damage: roll for damage as usual, then double it.
2. max roll: take the highest possible roll (but don't double it)

Lets say you are using a sword with d6 damage, and you roll a 20 on your attack roll. You get to choose:

1. roll your d6 and double it
2. don't roll, just deal 6 damage

The average of both these choices is 6 damage, but with option 1, you may deal anywhere in the range of 2 to 12 damage.

see Optional Rules

(You also have a 5% chance of rolling a “1” which is a Critical Fail)