Nidra means sleep. Yoga Nidra is the art of resting and sleeping.
These practices support not only sleep but are the best training for sitting meditation, as they train the nervous system to settle for long periods in a conscious restful state.
Yoga Nidra is also a traditional method of trauma recovery, as the third practice in this course explores. An exploration into pairs of opposite feelings and emotions: Comfort and discomfort, safety and fear, love and anger.

I recommend practicing the first and second sessions a good few times until you are familiar and comfortable with them, before trying the longer and more in depth third session.