Zero state = nothing, void, emptiness.
We all have idea of what void emptiness or nothing is. That idea also drops in zero because zero means zero. It cannot be seen it cannot do anything or it simply cannot exsist. It means absence of anything.
In simple terms in zero state everything is absence. Absence of anything is the proof of zero state.
How to know that Oneness is zero state? Because when you try to look and try to find out the essence of experiences and experiencer you will able to find nothing. Here you can see absence right? This absence of both is known as zero state.
From this absence created a pure infinite potential that pure potential is oneness the state where experiencer and experiences become one. Means you cannot find any difference between or any distance between the experiencer and experiences.
After this comes duality. This duality is also know as maya a.k.a illusion.
Why duality called illusion because it seperate the actual way exsistence is exsisting. The actual way exsistence is exsisting is based on oneness.
So it is more like this for the students of spirituality.
Zero state ( aagyata - state of not knowing).
↓
State of oneness. ( Dissolve of experiences and experiencer, simply can't able to find any distance or difference between experience and experiencer)
Here spirituality ends at stage of oneness because oneness and zero state are one thing they are not something different.
↓
State of duality ( awareness of self i.e. - experiencer and experiences knowledge)
↓
State of ignorance or pure illusion. ( This is state of ego - I, me, mine, my )
The above state of duality isn't illusion if you have self realisation. But in state of ignorance you are completely under control of illusion. In duality state, illusion is still there but you understand illusion but in this state you are under control of illusion.
So this is what spirituality journey looks like.
Zero state → Oneness state → Duality state → state of ignorance i.e. ego state.
Zero state - It is state of not knowing. Because now you cannot find anything. Like you can find essence of ocean, river, rain is water right? But when ask what is essence of water? You cannot able to find it right? If some smart one says no, essence of water can be molecules or then atoms then vibrations then something. Ask them what is the end of all essence? Now you could see right? End of all essence is zero.
Similarly when you cannot find essence of oneness that state is known as state of aagyata or zero state. You can see that oneness is also same as zero state. A bit different but yeah mostly same.