When you roll a number in your critical threat range (usually a 20), you threaten a critical hit. Check under 'Critical Hits' here. Also see this question. You then have to make another attack roll to confirm the critical. This roll, in general, consists of d20 + any modifiers that applied to your original roll against the enemy AC. If you make the confirm roll, the attack is a critical and does double damage. If you fail the confirm roll, the attack still hits, but doesn't count as a critical. This feat provides a substantial bonus, but only on the confirm roll, not on the original roll.
In your specific example, if you rolled a 16 it would not be a critical. If you required a 20 to hit (hypothetically), then a 20 would threaten a critical. When you rolled to confirm without the feat, a 20 would be required to confirm the critical (since you required a 20 to hit). However when you rolled to confirm with the feat, only a 16 or higher would be required to confirm.