We begin with the great ultimate.Some see the great ultimate as the Tao. Others see this as a representation of God. Others see it as "All that is" or "Source". It really doesn't matter what you call it. It is a way to help you frame and conceptualize how energy flows, changes, and moves. It also helps you see an order in chaos, or a way to help you understand how life is all connected.
In Chinese Medicine, this concept uses the construct of the yin and yang symbol to represent this higher power. The Tai Ji is also a representation of male-and-female-like subtle energies. This conception uses the color black for yin, or female-like energies, and white for yang or male-like energies.
By adding an additional line, which also helps us clarify the Tai Ji's position, we now have four possible images; Greater Yin, Lesser Yang, Lesser Yin, and Greater Yang.
The bagua is a tool we can use in Qigong, Martial Arts, Feng Shui, iChing, and even other modalities that help us understand and use life force energy. It provides a wonderful frame work to help us conceptualize how subtle energy moves, and then also helps us be aware and understand how this subtle energy can impact our lives and our health.