The Chinese character for acupoint is "xué" (穴), which translates into English as hole, cave or lair. The significance of this meaning can be found in the strokes of the character. The top of the character represents a "roof", and the bottom half refers to the number "eight". Together they refer to a hidden passage to the Eight Treasures, which are Viscera, Bowels, Qi, Blood, Sinews, Vessels, Bones, and Marrow.
In Chinese classical texts like, The Yellow Emperor's Cannon of Internal Medicine (Huangdi Nei Jing Su Wen), acupuncture points were described as specific sites where the Qi and blood from the zang-fu (organs) and meridians could reach the body's surface. This allowed access to the organ systems and the Qi & blood.
These specific sites were used to administer acupuncture treatments with needles of varying size, as well as treatments with other modalities such as, moxibustion, hot stones, cupping and guasha.
Acupoints were, and still are today, used as diagnostic sites which reveal internal states of disease. When an organ system becaomes excess/deficient or dysfunctional, the acupoint related to that organ will show morphological changes. In practice, acupuncturists will observe any changes in the appearance of acupoints, including; color or any type of dermatological changes, as well as, any temperature differences. The acupuncturist will then palpate along the meridians checking the points for any changes in resistance. when there is no resistance, and the finger falls into the acupoint, it's said to be deficient. When there is great resistance / the point is pushing back, there is excess.
Scientifically speaking, acupoints have some pretty interesting qualities that further illuminate how amazing our bodies are.
For instance, research has shown that acupoints are able to absorb and emit light and sound, hence the more modern addition of such modalities as laser acupuncture, and sound vibration through the application of tuning forks.
In sum, acupoints inherently have a dual function due to the fact that they are an opening, that connects the internal organ systems to the external world outside the body. This unique physical feature allows Eastern physicians to both diagnose the patient using observation and palpation, and most importantly, to treat the patient utilizing a wide array of therapeutic modalities.