Dream Yoga is the practice of lucid dreaming which is being aware that you are deaming when you are dreaming. Do this is usually quite difficult for most people but it is an important practice because it trains the mind to realize that all the outer phenomena are also "empty" or we can say an illusion or like a dream.

The second reason to practice dream yoga is that when one dies and enters into the bardo one almost always believes that one is awake and the visions of bardo are real and usually very frightening. However, if one has practice dream yoga, one can more easily recognize that one is literally in a dream created by our mind, not all these external visions created by something outside of us.

This book of 63 pages is fairly short and concise, but is a good place to start understanding dream yoga.