Ash Wednesday comes from the ancient Jewish tradition of penance and fasting. The practice includes the wearing of ashes on the head. The ashes symbolize the dust from which God made us. As the priest applies the ashes to a person's forehead, he speaks the words: "Remember that you are dust, and to dust you shall return."
Alternatively, the priest may speak the words, "Repent and believe in the Gospel."
Ashes also symbolize grief, in this case, grief that we have sinned and caused division from God.
Writings from the Second-century Church refer to the wearing of ashes as a sign of penance.
It is important to remember that Ash Wednesday is a day of penitential prayer and fasting. Some faithful take the rest of the day off work and remain home. It is generally inappropriate to dine out, to shop, or to go about in public after receiving the ashes. Feasting is highly inappropriate. Small children, the elderly and sick are exempt from this observance.
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.
Source: https://www.catholic.org/lent/ashwed.php