Yoga Nidra is a relaxation practice. The practice is done while lying on your back. You should remain completely still throughout the practice. Your physical body will go "to sleep" yet your mind will be awake during the practice. This will allow you to move your attention deep within your body.
By keeping your body completely still you may experience a state of bliss and may not feel your physical body at all -- what the yogi's call Samadhi.
The practice is used to reduce anxiety, insomnia, depression or to simply be more at peace with yourself and the world. Namaste'