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 with all the same modifiers as the attack roll you just made. And if the critical roll also results in a hit against the target's AC, the critical hit is confirmed and the base damage you inflict to your target is multiplied by your weapon's critical hit multiplier.

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 generally not rolled multiple times (such as 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.) Seeker weapons will improve the second roll (to confirm the crit) and also add to the base damage before the multiplier.