Ashes, as a Jewish sign of penitence, were accepted by Christians. They are derived from burning palms from the previous year.

This outward symbol of private or public sorrow, sadness, or penance, is a proof of humility, a remembrance of our mortality, that we are made of dust and will return to dust. The custom of imposing ashes, is a symbolic act signifying human mortality and total human dependence on the graciousness and mercy of God.

From the Congregation of Divine Worship (1988):

“On the Wednesday before the first Sunday of Lent, the faithful receive the ashes, thus entering into the time established for the purification of their souls. This sign of penance, a traditionally biblical one, has been preserved among the Church’s customs until the present day. It signifies the human condition of the sinner, who seeks to express his guilt before the Lord in an exterior manner, and…