When a character makes an attack roll and gets a natural 20 (the d20 shows 20), the character hits regardless of the target’s Armor Class, and the character has scored a critical hit.
A critical hit multiplies the character’s damage. Unless otherwise specified, the multiplier is x2. When you multiply damage more than once, the multiplier increases by one (e.g. two x2 multipliers become x3, three become x4, and so on).