A natural 20 - rolling a perfect twenty on a twenty sided die is a success, no matter what you are rolling for. this is also called a "critical" success, in that it succeeds in an amazing and awe inspiring well (critically, if you will)
for instance, if you roll a natural twenty to percieve any trace of where the bear you're hunting has gone, you might find tracks leading you straight to his den, or you might even percieve his location and true, astral identity, becoming one with and instantly attaining all of his attributes.
a critical failure (natural one) is the exact opposite - rolling a natural one to percieve where the bear has gone could cause you to a) fail to find him, get yourself lost in the woods b) cause you to go blind c) you're the bear, and the hunter that was hunting you just killed you
DMs can Genie or guardian angel your rolls (natural 1, you slip and kick this bitch in the jaw, killing him instantly. natural 20, you hide so well you can't find yourself)