Khushu means calmness, serenity, tranquility, dignity and humility.
One gains khushu from a fear of Allah and the sense that He is always watching. It means that one’s heart stands before its Lord with humility and submission. Allah tells us:
“…and stand before Allah with obedience.” (Surah al-Baqarah 2:238)
Part of obedience is to bow, to be solemn and submissive, to lower one’s gaze and to humble oneself out of fear of Allah, may He be glorified.
The site of khushu is in the heart, and its effects are manifested in the physical body. The various faculties follow the heart: if the heart is corrupted by negligence or insinuating whispers from Shaitan, the worship of the body’s faculties will also be corrupt.
The heart is like a king and the faculties are like his troops who follow his orders and go where they are commanded. If the king is deposed, his followers are lost, which is like what happens when the heart does not worship properly.