I don't know any specific explanation, but I'll tell how I understood it.
We have three states of Consciousness the first one is Consciousness itself which comes into effect when we are awake. The second one is unconsciousness which will be alive when we are asleep.
But the third one is strange. It is subconsciousness and it takes over whenever one starts to dream. It is very powerful, capable of manipulating our thoughts, or even sum them up to give an arbitrary thought/feeling.
When go to sleep and still thinking about something then you are trying to go to unconsciousness state but not leaving the consciousness state. Which automatically leads you to subconsciousness state. Now here you can imagine things and mould them. Sometimes the subconsciousness itself gives you dreams that might come true (you know sometimes when you go to a place/enter a situation, you feel that you have come across it? Actully you have came across it, in one of your subconsciousness states. So sometimes with the help of subconsciousness you can actully predict things!)
There is one movie by chistophor Nolon, it is Inception. A nice movie.