Natural 20 - Critical Hit or perhaps just "Crit". A natural 20 generally means you succeed on a hit or whatever you're attempting to do, and if, in combat, you roll a 20 and you beat the defending creature's AC you will deal max damage with your attack.
Natural 1 - Critical Fail or just "Fail". Rolling a natural 1 means you've probably going to fail at whatever you just attempted to do, even if you are a master of the craft that you are attempting to complete. Some Dungeon Masters see fit to add additional penalties when this happens during battle such as shooting yourself in the foot with a bow and arrow, or dropping your melee weapon, etc.