“Jing” translated from Chinese is Essence. Along with Qi - the life force, and Shen - the spirit, Jing - the essence, is considered one of the “three treasures” in traditional Chinese medicine. These “three treasures” are the substances that nurture the organs and organ systems and keep the mind, body, and spirit in balance.
The Essence determines the growth, development and maturation, and is responsible for the vitality and longevity in every person. There are two types of Essence - prenatal Essence, which is acquired from our parents (some compare it to DNA), and postnatal Essence, which is gathered during life through the drinks and food we intake.
The Essence is stored in the Kidneys. If the Kidneys are weak or injured this will effect the Essence. To improve the Essence, i.e. to imrpove one's vitality and longevity, one needs to improve the health of the Kidneys (for causes, symptoms and treatment of Kidney depletion you can review the materials about the Kidney in the Physiology chapter)