Summary: The meaning of the Cube of Space is lost if it is not understood as a description of our own psychological reality. The space in the Cube of Space is our own psychological space and its dimensions are the psychological dimensions of our own experience. We can easily find its "common-sense" descriptions in our own experience and verify their accuracy or usefullness for ourselves.
The Cube of Space describes the human reality of Adam Qadmon, archetypal man, as composed of an inner life formed in the first four Sephirot (spheres of energy) and the axes and center of the Cube, and an outer life formed by the last six Sephirot and the opposing faces of the Cube. The inner life is timeless and the outer life is experiential and developmental.
Inner space or being is formed by three primary energies, represented by the Hebrew "Mother" letters: Aleph, maximum energy/consciousness, Mem, minimum energy/consciousness, and Sheen, the energy mediating between them. Outer space or being is the reflection ofthose energies as existence, life and experience. Existence is formed by the polarity of self and other, life by the polarity of future and past, and experience by the polarity of feeling and thought, or stimulus and response. The psyche itself, the seventh direction of the Cube, is at the center, though which all the energies pass.