The dimension can only be accessed by going to a deep sleep, and the dimension itself is controlled by the Moon.
The Dreamworld is mainly made of an empty blue void, but it is surrounded by stars. It is inhabited by children, Zodiac constellations and Animal Totems who would enter the dimension and sing magical songs to the children.
In the Series
In "Les Pyjamasques et Roméo Mécano", Les Pyjamasques and Ptigarou had to protect the dimension by securing the Faced Moon that keeps the Dreamworld stable from Roméo Mécano using one of his inventions to cut the Faced Moon to pieces, therefore attempting to make people not dream anymore. However, Yoyo managed to turn Ptigarou back into Grogarou to foil the villain's plans.