Ash Wednesday is the first day of Lent. The name comes from the tradition of putting a small cross of ash on peoples forehead at Ash Wednesday church services. This is a sign of confession and helps people to remember that they rely on God for forgiveness from their sins. It is still done in some churches on today. The Ash sometimes comes from palm branches that were burned after the Palm Sunday service the previous year.
Ash is used because it is a symbol of death, because all bodies turn to ash or dust in the end. It helps Christians to remember that Lent is about preparing to remember that Jesus died.