The act echoes the ancient Near Eastern tradition of throwing ashes over one's head to signify repentance before God (as related in the Bible). The priest or minister says one of the following when applying the ashes:

Remember, O man, that you are dust, and unto dust you shall return. (Genesis 3:19)

Turn away from sin and be faithful to the Gospel. (Mark 1:15)

Repent, and hear the good news. (Mark 1:15)

In the Roman Catholic Church, Ash Wednesday is observed by fasting, abstinence from meat, and repentance—a day of contemplating one's transgressions.
On Ash Wednesday and Good Friday, Roman Catholics between the ages of 18 and 59 are permitted to consume only one full meal, which may be supplemented by two smaller meals, which together should not equal the full meal. Some Roman Catholics will go beyond the minimum obligations demanded by the Church and undertake a complete fast or a bread and water fast.
Ash Wednesday and Good Friday are also days of abstinence from meat (for those Catholics age 14 and over), as are all Fridays in Lent.
Some Roman Catholics continue fasting during the whole of Lent, as was the Church's traditional requirement, concluding only after the celebration of the Easter Vigil.

As the first day of Lent, Ash Wednesday comes the day after Shrove Tuesday or Mardi Gras (Fat Tuesday), the last day of the Carnival season.