Xu refers to a state of the cosmos or a state of mind. Basically, it has two different meanings. The first refers to the origin of the universe, indicating that everything originates from xu. Different ancient thinkers have different interpretations of this notion: Some take xu as being devoid of anything; others believe it is the state of existence of qi (气). Because qi is invisible and formless, it is said to be empty, but not a vacuum totally devoid of anything. The second meaning of xu refers to a state of mind that is peaceful, not preoccupied or simply free of any preconceptions.
Xu is formless; it is the original state of qi. (Zhang Zai: Enlightenment Through Confucian Teachings)
Dao gathers and presents itself in an unoccupied and peaceful mind; being unoccupied means the pure state of the mind. (Zhuangzi)