A cube is the Earth, the material world, the four Elements, the foundation of the cosmos. Cubes represent wholeness and also Truth because the view is the same from every angle. Cubes also symbolize solidity and firmness and in allegories represent the persistence of the virtues. Chariots and thrones often are represented as cubes.
In alchemy, the cubes is the squaring of the circle, or completion and wholeness. It also represents salt.