The Yin and Yang symbol originated from Chinese philosophy, which stated that there was an Undifferentiated Oneness (Taiji, the Supreme Ultimate) prior to the appearance of the opposing forces of duality. It's a symbol commonly used in Taoism. There are a lot of similarities with Taoism and Non-Duality. But here I'm going to look at Yin and Yang from the Non-Duality perspective.
It might seem odd for someone reading for the first time that the Yin and Yang symbol relates to an Undifferentiated Oneness. They are opposites, right? So what's that got to do with Oneness. Well, in Non-Duality we recognise that the world appears as an interplay of opposites, but this is illusory in nature. The True Reality (or I could say, the Supreme Ultimate Reality) is Infinite Indivisible Oneness.
The Yin and Yang symbol shows the interplay of opposites enclosed in a circle. The circle represents the infinite oneness. I realise that my books have been about this all along. In the books I explain about The World of Opposites and The Law of Opposites, but I also point out that the Reality is not two. The Reality is the Infinite One. I liken Reality to an Infinite Singularity, the original unmanifest state of being. This sounds a lot like the Undifferentiated Oneness of Taiji, the Supreme Ultimate.
The Yin and Yang symbol (called Taijitu) is a wonderful visualisation of the interacting, flowing forces of the world. The opposites appear as a clear reflection of each other, light and darkness. The opposing forces flow in unison, from one to the other, and one cannot appear without the other appearing. There is an underlying sense that these opposites remain as one.
As I've explained elsewhere, The World of Opposites is an appearance of separation within the Infinite Singularity of Being. It's as if the Infinite Undifferentiated Singularity exploded into a world of opposites, what is like it and what is not like it, what is and what is not. That really seems to be a natural way to make the invisible Oneness visible - by use of opposites or reflection. It seems that what first appears is 'what is' against 'what is not'. Imagine a point of light appearing in the darkness. The point is 'what is' and the darkness is 'what is not'. A duality seems to be created, and this basic duality can combine to form the complex World of Opposites that we experience. It might seem like a huge leap, but imagine how video games have developed from the basic 1 and 0. A world of complex interactions can be developed from this basic dichotomy.
It may seem like a dichotomy - a splitting into two opposites - but it's not really. You see, the Oneness is still here. The Infinite Singularity cannot really be divided in two. It is timeless and unchanging. There is no space or time in the primordial Singularity. So when space and time appear they appear within this Infinite One. This is where we reach over to Hindu philosophy, and the Upanishads, that teach that the Ultimate Reality is our own Aware Being. This Infinite One is who we are. There is none other than the Infinite One. It is the Infinite Consciousness that experiences the world as an interplay of opposites.
Our true nature is not a conflicting or interacting of opposites. We are truly beyond the appearance of the world. The Infinite One cannot truly see itself. Only by aid of reflection and opposites can the Infinite One experience itself as limited forms. The Formless perceives forms. The Changeless perceives change. Yet there is no outside or inside of Infinite Consciousness. The play of separation of opposites appears within the Infinite One, the Ultimate Reality, Pure Consciousness.