From Catholic Online:
Ash Wednesday marks the beginning of the Season of Lent. It is a season of penance, reflection, and fasting which prepares us for Christ’s Resurrection on Easter Sunday, through which we attain redemption.
Following the example of the Ninevites, who did penance in sackcloth and ashes, our foreheads are marked with ashes to humble our hearts and reminds us that life passes away on Earth.
Ashes are a symbol of penance made sacramental by the blessing of the Church, and they help us develop a spirit of humility and sacrifice.
The distribution of ashes comes from a ceremony of ages past. Christians who had committed grave faults performed public penance. On Ash Wednesday, the Bishop blessed the hair shirts which they were to wear during the forty days of penance, 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 church because of their sins — just as Adam, the first man, was turned out of Paradise because of his disobedience. The penitents did not enter the church again until Maundy Thursday after having won reconciliation by the toil of forty days’ penance and sacramental absolution. Later, all Christians, whether public or secret penitents, came to receive ashes out of devotion. In earlier times, the distribution of ashes was followed by a penitential procession.
The ashes are made from the blessed palms used in the Palm Sunday celebration of the previous year. The ashes are christened with Holy Water and are scented by exposure to incense. While the ashes symbolize penance and contrition, they are also a reminder that God is gracious and merciful to those who call on Him with repentant hearts. His Divine mercy is of utmost importance during the season of Lent, and the Church calls on us to seek that mercy during the entire Lenten season with reflection, prayer and penance.