The basic meaning of Khushu is the softness of the heart, its being gentle, still, submissive, broken, and yearning. The source of Khushu that takes place in the heart is the awareness of Allah’s Greatness, Magnifence and Perfection. The more awareness one develops, the more Khushu he has.
Ibn Rajab Al Hanbali wrote a treatise on developing Khushu in Salah, a short book beautifully translated into English. Learn deep lessons that will affect your prayer and your every day life.