Jing - translated as Essence, a very precious substance, should be guarded and not wasted.

Prenatal Jing (Pre-Heaven Essence)

At conception, the Prenatal Jing is passed to the fetus from the parents.

Prenatal Jing (together with an energy derived from the Kidneys of the mother) nourishes the fetus during pregnancy.

Prenatal Jing determines basic constitution, strength, and vitality. It is fixed in quantity, determined at birth: it cannot be added to, only conserved and used up more slowly. It is stored in the Kidneys.

The way to conserve Prenatal Jing is by striving for balance in all life activities. Balance meaning moderation in diet, work/rest, sexual activity. Irregularity or excess in these areas wastes Prenatal Jing.

Certain exercises help conserve Prenatal Jing, such as breathing exercises, Taiji (Tai Chi), and Qi Gong.

Postnatal Jing (Post-Heaven Essence)

After birth, the child begins eating, drinking, and breathing independently. Its Lungs, Spleen and Stomach then begin functioning to extract and refine Qi from the food and drink it consumes and the air it breaths. Postnatal Jing is the complex of essences thus refined and extracted. Postnatal Jing depends on the functions of Stomach and Spleen.

Jing, translated as Essence, is a very precious substance, which should be guarded and not wasted.

Two main kinds: Prenatal Jing (Pre-Heaven Essence) and Postnatal Jing (Post-Heaven Essence).

Kidney Jing

Kidney Jing plays important role in physiology.
Is hereditary, like Prenatal Jing and determines constitution.
However, is partly replenished by the Postnatal Jing.
Stored in the Kidneys but has fluid nature and circulates all over the body, especially in the Eight Ancestral (Extraordinary) Vessels.
Forms the basis for growth, development, sexual maturation, and reproduction.
Moves in long, slow cycles, and presides over the major phases of development in life.

The first is in childhood. Kidney Jing controls growth of bones, teeth, hair, brain development and sexual maturation. Where Kidney Jing is weak, there may be poor bone and teeth development, stunted growth, and mental retardation.
Puberty. Kidney Jing controls reproductive function and fertility, and normal development into adulthood. Developmental problems that occur at this time such as amenorrhea, are often related to weak Kidney Jing.
Conception and pregnancy are guided and controlled by Kidney Jing. Where Kidney Jing is weak, signs such as infertility, chronic miscarriage and other such problems may occur.
Kidney Jing declines naturally, finally producing the characteristic signs of hair/teeth loss, impairment of memory, etc.

Kidney Jing is the basis for Kidney Qi

Jing is fluid-like and therefore more Yin. Can be considered as an aspect of Kidney Yin.
Forms the material basis for production of Kidney Qi (via the warming action of Kidney Yang. Kidney Yin is warmed by Kidney Yang and Kidney Qi is formed, but Kidney Jing is necessary before this transformation can occur.
Deficiency of Kidney Jing can thus result in such problems as impotence, chronic weak or sore lower back, weak knees, tinnitus, urinary incontinence, deafness, loose teeth, etc. (all signs of weak Kidney Qi and/or Yang.)

Kidney Jing produces Marrow

Kidney Jing produces Marrow. Marrow produces bone marrow, the brain, and the spinal cord (Marrow in TCM has no exact equivalent in Western Medicine).
The Brain in TCM is called the "Sea of Marrow". Therefore if Kidney Jing is weak, brain may be undernourished, leading to poor memory/concentration, "empty" feeling in the head, dizziness, etc.

Kidney Jing determines our Constitution

Our protection from exterior pathogens depends largely on the strength of a type of Qi: the Defensive (Wei) Qi.
However, the state of Kidney Essence also influences our strength and resistance. If the Essence is "wasted" or poorly stored, the person may have lowered immunity to exogenous pathogenic influences and constantly be ill with cold, influenza, allergies, etc.

Essence and Qi are the material foundation for Shen (Mind).