The Void is the rare hole in the bedrock that allows players to fall through the designated 'bottom of the map.' Originally a glitch in the coding that spawns in bedrock, Notch later kept the void as a feature instead of a glitch.
The color of the Void changes between day and night, players can use this to determine the time of day by covering it with glass. If the player drops into the Void, they will lose health at a rate of 3 hearts per second, and cannot exit once fallen in. Also you cannot obtain your items back when you die because the player has fallen off the map.