Jing could be the internal sources or structure base of Qi

In order to understand the concept of Qi, we need briefly discuss another related TCM concept "Jing". Jing is usually translated as "Essence". The Chinese character implies that it is a refined substance derived from a coarser one. In many senses, Jing could be the internal sources or structure base of Qi. Jing itself can be divided into different types or be looked from different angles. If Qi is used in the sense of function, Jing would be understood as the physiological structure. If Qi is considered as vital energy, then Jing would be the physiological systems that support the energy. For example, endocrine system is frequently referred as "jing" in TCM. Keep it in mind that there are disagreements on what can be called Jing, what can not. Basically there are three different types of Jing discussed in TCM classic books. 

Prenatal Jing (Pre-Heaven Essence)
At conception, the Prenatal Jing passes from the parents to the embryo. This essence, together with nourishment derived from the Kidneys of the mother, nourishes the embryo and fetus during pregnancy. It is the only kind of essence present in the fetus.

Prenatal Jing determines basic constitution, strength, vitality, and so individual uniqueness. Since Prenatal Jing is inherited from the parents, it is very difficult to influence in later life. Some say the quality and quantity of Prenatal Jing cannot be altered. The way to conserve Prenatal Jing is by striving for balance in all life activities – moderation in diet, work/rest, and sexual activity. Irregularity or excess in these areas wastes Prenatal Jing. Certain exercises help conserve Prenatal Jing, such as Tai Chi and Qigong. Tortoise breathing may also positively influence it. 

Postnatal Jing (Post-Heaven Essence)
After birth, the infant starts to eat, drink, and breathe on its own. The Spleen and Stomach then extract and refine Qi from the food and drink and the Lung gets Qi from the air. Postnatal Jing is the complex of essences thus refined and extracted. It is the material basis for the functional activity of the body’s internal organs and metabolism. The Kidneys store any surplus Jing to be released when requirestnatal Jing is continually being used by the body and replenished by food and drink. The Prenatal Jing is enriched and functions optimally only through the action of the Postnatal Jing. Without the function of the Prenatal Jing, the Postnatal Jing cannot be transformed into Qi.