The imposition of ashes on our foreheads at the beginning of Lent is a very old practice that the Church conserves in her liturgy. In the 4th century, when the persecutions of the Church had ended, the Worship and administration of Sacraments was established in a more organized manner. At this time, you could only receive the absolution of your sins once in a lifetime. Those who had gravely sinned had to do public penance, declare themselves repentant sinners and prove their desire to receive God’s forgiveness.
During Lent, they would receive the imposition of ashes on their heads and entered the “Order of Penitents”—that is, they joined those who did penance during Lent to be able to receive the absolution of their sins in the Easter season.
That is the meaning of the imposition of ashes. Today, we also recognize ourselves as sinners and receive the ashes as a sign of our desire of conversion, of being reconciled with God and to grow in our union with Him. We know that God loves sinners who humbly recognize their sin. As Psalm 50 says: “A broken and contrite heart, O God, you will not despise.”
Answer given by Fr. Félix López, S.H.M.