In a dream you are there and this is the world I'm experiencing. Our dreams are exactly like that. It's only because we have woken up from them when we can dismiss them as a dream. World of dreams is nothing but what you experience in dreams. What whatever we have experienced in the waking state, the materials are taken gathered there, stored in our minds. And those are cooked up as dreams.
Swami Sarvapriyananda teaches one of the shortest and most profound upanishads accompanied by Gaudapada's Karika. This lecture covers Karika 1-5.