Realm of Toril
Critical misses occur when a 1 is rolled on the attack roll. When this happens it goes beyond your general miss, where the opponent has simply blocked or evaded your attack. With this, something bad has happened. Maybe you lost your footing and stumbled, maybe your grip failed and your weapon flung out of your hand. When a critical miss happens, the DM will roll 1D20 to determine just what exactly the consequences will be of the roll.
14-20: Attack just misses
10-13: Stumble, -1 to all attack rolls the next round
6-9: Roll a dexterity check or lose your weapon
2-5: DM gets creative1
1: …DM rolls again, the DM gets to be very creative based on how low the roll is.
1 This is NEVER a good thing. The DM is a cruel, sadistic being who enjoys watching Fighter fall into holes.