The Nexus is a single room located through Urotsuki's dream bedroom door. It is the main hub for all of the different area in the dream world.
By using the Medabomb effect in any of these worlds, you can teleport back here without having to wake up.
At the moment, there are 13 external worlds accessible from the Nexus.
Some areas cannot be accessed directly from the Nexus, meaning that you will need to explore a lot in order to find all of the effects.