Yoga nidra, or yogic sleep, is a state of conscious relaxation. The aim is to enter a state of near sleep where bodily function slows down and the powerful anti-oxidant melatonin increases. This is accomplished by lying down in what is referred to as the “Shavasana” and quieting your mind, usually in a silent, softly lit room.
If you are new to the practice there are lots of resources you can use to get yourself started, including this one (link).