What is the meaning of [Man does not live by bread alone]

Physical nourishment is not sufficient for a healthy life; man also has spiritual needs.
From the Bible, Deuteronomy 8: 2-3 (King James Version):

And he humbled thee, and suffered thee to hunger, and fed thee with manna, which thou knewest not, neither did thy fathers know; that he might make thee know that man doth not live by bread only, but by every word that proceedeth out of the mouth of the Lord doth man live.