When a thought is present, you know it. When a thought is not present, what is it that knows that the thought is not there?
Nonduality (or advaita) is a deep and earnest inquiry into self-knowledge. The literal meanings of "not two" and "oneness" are often given. Both definitions are decent enough. But in essence, we aren't speaking numerically here. We are saying that there is no subject and object. There is only awareness, which is the ultimate Subject.
There is no progression to truth, just a sudden seeing or understanding. There is sometimes a period where one's preconceptions about this natural state are dealt with and cleared away. But throughout it all, presence remains vibrant and clear.
We are taught that consciousness is the epitome of existence. But if you will notice, you are aware of being awake as you read these words. Consciousness is occurring inside of that sheer and consummate spaciousness, which is none other than yourself.