Acupressure involves placing physical pressure, by hand, elbow, or with the aid of various devices, on different pressure points on the surface of the body (which may be far distant from the symptom, related by what is called the meridian system) to bring about relief through greater balance and circulation of fluids (blood, lymph) and metabolic energies in the body (heat, qi). It is intended to help balance the systems of the body in order to treat chronic and acute injuries or disease. However, the theory of acupressure can be irrelevant if you are simply looking for a massage and do not care to differentiate between massage styles.
The first formally recognized mention of qi is in the Chinese writings of the Shang dynasty oracle bones which were produced between the 16th to 11th century B.C.
Possibly the earliest evidence of use of the meridian system for health purposes has been found in Europe. Ítzi the Iceman, a 5,000 year old mummy found preserved in an Alpine glacier, seems to have tattoos, some of which correspond to points that a modern acupuncturist or tui na specialist would use to treat symptoms of diseases that Ítzi seems to have suffered from, including digestive parasites and degenerative bone disease. However, it is difficult to speculate on the meaning of the tattoos as to whether they served a decorative, religious, or a medical function.