Xu means empty or void. The Daodejing counsels reaching the utmost emptiness and holding fast to stillness; the Zhuangzi describes the Dao gathering in emptiness. It is the mind free of preoccupation, receptive like a valley, able to respond to things as they come.
Xu
Emptiness or tenuousness: the empty, receptive state of the mind that the Daoist sage cultivates.
Also: hsu, emptiness