Basically, it is a way of reaching a state of deep meditation but in the lying down or savasana position and therefore avoiding the typical discomforts.
Yoga nidra, a Sanskrit term meaning "yogic sleep" is a technique of deep relaxation and a form of meditation.
Also called "psychic sleep", yoga nidra is a state between sleep and wakefulness. The body is completely relaxed and the practitioner turns the consciousness inward by listening to a series of instructions; much like a guided meditation. Performing yoga nidra involves practicing pratyahara ("extraction of the senses"), which is the fifth branch of Ashtanga yoga.
It is derived from the ancient tantric Nyasa practice. It is an important technique of meditation and deep relaxation developed by Swami Satyananda Saraswati who adapted and updated these ancient practices, preserving their essence but removing the ritualistic complications, adapting them to our time and making them more accessible to non-initiated people.
The practice of yoga nidra produces deep relaxation and expands the individual's self-awareness, as well as important benefits on all levels:
Calm the mind
Relaxes and rejuvenates the body
Relieves the nervous system
Reduces high cholesterol and blood pressure levels
Improves sleep quality
Improves the level of concentration
Supports brain function and increases creativity
One hour of Yoga Nidra is equivalent to 4 hours of deep sleep. I discovered this by "causality" in my first Vipassana Meditation retreat and it changed forever the relationship I have had with insomnia throughout my life.