Jesus' call to conversion and penance, like that of the prophets before him, does not aim first at outward works, "sackcloth and ashes," fasting and mortification, but at the conversion of the heart, interior conversion. Without this, such penances remain sterile and false; however, interior conversion urges expression in visible signs, gestures and works of penance.
Source: Catechism of the Catholic Church (Vatican.va)