Normal awareness — which I will simply called “consciousness” here — involves the perception of a division or distinction between the subject and the object of our awareness. That distinction is among the most fundamental characteristics of consciousness.
Most of us — whether we have ever thought about it or not — are so accustomed to that distinction that if given the opportunity we would probably swear it was intrinsic to the structure of reality itself. It seems, however, that cannot be demonstrated to be the case. In other words, our epistemologies are not powerful enough to allow us to say with certainty whether the subject/object distinction we consciously perceive is a property of reality, or rather a mere effect of consciousness.
If the distinction is intrinsic to reality, then the universe is comprised of a nearly infinite number of discrete things. But if the distinction is merely an effect of consciousness, then the universe is — in some way we have yet to understand — ultimately one thing.
See also The Study of Mystical Awareness