The Edge of Eden is the area of Eden where the world abruptly stops generating. Any blocks placed beyond this point will replace the block within the world whose block face you attempted to place it on. Walking into this area will cause the player to fall to the bottom of the world just below bedrock and instantaneously teleport back up to the highest layer it is possible to build in. This effect also works for monsters. If you want to see the Edge of Eden search up "Far Lands" in the Shared Worlds section and download the world called "eden far lands end of the world".
The Eden Far Lands creates bad chunks for a large amount of land near the edge of the world. It is unknown exactly how close to the Far Lands you will have to get, but you will know it when you see it. These chunks consist of incredibly rectangular and imprecise terrain.
An interesting quirk about this area is that because Bedrock has no bottom texture, you can see through the world for a moment before being teleported back up above the world.