It is an action that goes all the way back to the Old Testament. When someone felt true contrition for their sins, they put on sackcloth (rough, coarse garments) and either poured ashes over their heads or sat in them (see Dan.9:3; Jonah 3:5-6). Jesus made reference to the practice in Mt.11:21.
Throughout the early Church, when someone returned to the practice of the Faith after a grave sin (adultery or apostasy, for example), ashes were sprinkled upon their heads. By the 11th century, it became the practice for all Christians to have ashes applied to their foreheads to mark the beginning of Lent, the time when all of our minds should turn to acts of penance. Here in North America, the usual practice is for the priest to make a kind of paste by adding holy water to the ashes and to apply them to our foreheads in the Sign of the Cross with the words, "Turn away from sin and be faithful to the Gospel."
Immediately after receiving the cross of ashes on my forehead, my mind went to a passage from Ezekiel. For me, it is one of the great prophetic images of how Jesus would save us. The context is the fall of Jerusalem to the Babylonians in 586 B.C., but it is applicable to all of us who mourn for the world's sins and look forward to Christ's promise of salvation. Meditate on this:
I saw six men coming from the direction of the upper gate [of the Temple] which faces the north, each with a destroying weapon in his hand. In their midst was a man dressed in linen, with a writer's case at his waist. They entered [the Temple] and stood beside the bronze altar. Then he called to the man dressed in linen with the writer's case at his waist, saying to him, “Pass through the city and mark an X on the foreheads of those who moan and groan over all the abominations that are practiced within it.” To the others I heard him say, “Pass through the city after him and strike! Do not look on them with pity nor show any mercy! Old men, youths and maidens, women and children--wipe them out! But do not touch any marked with the X; begin at my sanctuary” (Ezekiel 9:2-6, NIV)What an honor to be marked with the sign of Jesus' Cross!