Critical hit

If you hit your target while getting a (non-modified) d20 roll within your weapon's threat range, this results in a critical threat. An additional attack roll is made, and if it results in a hit, the critical hit is confirmed and the base damage you inflict to your target is multiplied by your weapon's critical hit multiplier. Note that the second roll does not need to fall within the threat range, it just needs to be enough to hit your target (the exact same attack bonus as your initial attack is used).

The base damage is not exactly multiplied, but rather rolled for every critical hit multiplier. For instance a +2 longsword doing 1d8 + 2 damage on a normal hit will do 2d8 + 4 damage on a critical hit since it has a 2x critical hit multiplier. On another hand, a +3 greataxe doing 1d12 + 3 damage on a normal hit will do 3d12 + 9 damage on a critical hit, because it has a 3x critical hit multiplier. Your strength bonus is also counted as part of the base damage, and thus multiplied by the critical hit multiplier.

Additional damage is in general not rolled multiple times (like elemental, alignment, bane, or sneak attack damage), except when specified in the weapon's effect description (for instance burst weapons do additional elemental or alignment damage on a critical hit).