Phenomena, such as thoughts or physical objects, are said to be ‘empty’ in the sense that they have no inherent, independent substance or existence of their own. In other words, they are not made out of something called ‘mind’ or ‘matter’ that exists in their own right independent of Awareness.
The essential substance or reality of all experience is what is sometimes referred to as Awareness, God, Presence or ‘I,’ and the apparent reality of the mind, body and world could be said to be modulations of this one essential substance or reality.
For instance, in a movie the characters, houses, fields, trees and sky are not made out of flesh, bones, bricks, soil, wood and air. They are made of the screen. Their essential substance or reality is the screen.
The characters, houses, fields, trees and sky are not real as objects – as objects they are empty, void, nothing (not-a-thing) – but as screen they are real. In fact, the characters, houses, fields, trees and sky are not real as screen but rather only the screen is real. There is only the screen. The characters, houses, fields, trees and sky only appear to be real from the illusory point of view of one of the characters.
Likewise in experience only Awareness is real. The apparent reality of the mind, body and world is imagined with the thought that thinks it. In other words, the constructs of thought, that is, the beliefs we have about the mind, body and world - are only real for thought itself.