The sacrifice acceptable to God is a broken spirit; a broken and contrite heart, O God, thou wilt not despise (Psalm 50.17)
Following the example of the Ninevites, who did penance in "sackcloth and ashes", today we humble ouselves by going to a priest, who will sign our foreheads with ashes both to humble our hearts and to remind us not only of our mortality on Earth, but also that the only Redemption is with Our Lord.
Ashes are a symbol of penance made sacramental by the blessing of the Church to help us develop a spirit of humility and sacrifice. The custom is from an old ceremony. Christians who had committed grave faults were obliged to do public penance. On Ash Wednesday the Bishop blessed the hairshirts which they were to wear during the forty days, and sprinkled over them ashes made from the palms from the previous year. Then, while the faithful recited the Seven Penitential Psalms, the penitents were turned out of the holy place because of their sins, as Adam, the first man was turned out of paradise on account of his disobedience. They did not enter the Church again until Maundy Thursday after having won reconciliation by the toil of forty days' penance and sacramental absolution. Later on, all Christians, either public or secret penitents, came out of devotion to receive ashes.