| Basically it means with your chosen weapon for the feat, any time that you make a critcal threat (Note, when criticalling, you roll 2 rolls, 1 for the threat which is usually rolling a 20 on the dice roll, and then another to confirm which requires that you merely hit the opponents AC again), you will gain a +4 to confirm that critical thread.
So if you have a Heavy mace as your power critical weapon, and you have a +4 to attack. If you roll a 20 automatically, your next attack roll to confirm that critical will be +8 (+4 naturally, and +4 from the power critical feat to aid in criticalling).
If you're going for a fighter or mix of fighter and some other tank class like barbarian, this is definitely a worth while feat. | 
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