The Void is the big empty space under each world, gazing through spaces in Bedrock at the bottom of the map.
This space is normally light blue in colour as it is always daytime in the Void; however, falling into this space will cause the player's health to rapidly decrease, and eventually kill the player. Upon respawning, the player will have lost all held items - with no ability to regain them.
The Void, found on the bottom of the map.