Non-duality is the understanding that, ultimately, there are no separate things in existence.
The appearance of separate objects is a projection of the mind - something that provides a way for us to interact with the world, by slicing it up into beings, things, times and places, but isn't actually a true representation of the way things are.
In meditation, we can unearth these projections of separateness, becoming familiar with them, and begin to see them as just constructions. And with long-term commitment to meditation practice (or a does of good luck), we can, at times, experience moments without the projection of a boundary between "me" and "not me". We can open to a sense of great mystery - the unfathomable existence of consciousness, immediate and pristine.
This is truly the deeper end of meditation practice, but it is not by any means beyond the reach of dedicated practitioners. And neither is it a final destination. If there are no real, separate, definable things (as non-dual spiritual teachings would hold) there are no limits on how this mystery of being can be experienced and understood. We can be just ever more deeply immersed in the wonder of being alive in a universe we can never understand.