To cloth oneself with sackcloth, a course and uncomfortable material, and place ashes over oneself is to publicly demonstrate self-humiliation. This is the ancient symbol of complete humility before the Lord.
Bruce R. McConkie
"The use of sackcloth and ashes anciently was also a token of humility and penitence. When righteous persons used the covering of sackcloth and the sprinkling of ashes to aid them in attaining the spiritual strength to commune with Deity, their usage was always accompanied by fasting and prayer. Daniel, for instance, prefaced the record of one of his great petitions to the throne of grace with this explanation: 'I set my face unto the Lord God, to seek by prayer and supplications, with fasting, and sackcloth, and ashes: And I prayed unto the Lord my God, and made my confession.' (Dan. 9:3-4; Isa. 58:5; 1 Kings 21:17-29.)" (Mormon Doctrine, p. 659)