When a character makes an attack roll and gets a natural 20 (the d20 shows 20), the character hits regardless of the target’s Defense, and the character has scored a threat of a critical hit. To find out if it is actually a critical hit, the character immediately makes another attack roll with all the same modifiers as the attack roll that scored the threat. If the second roll also results in a hit against the target’s Defense, the attack is a critical hit. (The second roll just needs to hit to confirm a critical hit; the character doesn’t need to roll a second 20.) If the second roll is a miss, then the attack just deals the damage of a regular hit.
A critical hit multiplies the character’s damage. Unless otherwise specified, the multiplier is x2. (It is possible for some weapons to have higher multipliers, doing more damage on a critical hit.) Some weapons have expanded threat ranges, making a critical hit more likely. However, even with these weapons, only a 20 is an automatic hit. The Critical column on the weapon description pages indicate the threat range for each weapon on the tables.
Bonus damage represented as extra dice is not multiplied when a character scores a critical hit.
Objects (including vehicles) and some types of creatures are immune to critical hits. A 20 is always a successful hit, but deals no extra damage against these targets.