The Devils Fruit is plot device from the anime and manga known as One Piece.
In short, these fruits are supposed to give people a specific super power when eaten. These powers differ vastly, but most seem to be pretty bizarre.
A side effect of eating a Devils Fruit is that one loses the ability to swim. In the setting of One Piece this is quite the problem, as most of the world is covered in water. Most towns exist on small islands and people have to use boats to get from place to place.
