First of all the ash is made from the burning of last year’s palm crosses given out on Palm Sunday when Christians remember Jesus’ triumphal entry into Jerusalem. This is symbolic of where the season of Lent is heading to - Ash Wednesday is the start of this penitential season and Christians are look forward to Holy Week which begins with Palm Sunday.
Finally Christians might like to reflect on the power of ash - out of such an apparently dead thing comes new life. Ash is a fantastic fertilizer and this seems to be a perfect metaphor for the message of Easter when Christians believe that Jesus used his death to secure new life for the world in the resurrection. Christians remember how in a few short weeks they will celebrate that in Jesus’ death and suffering their future salvation has been bought.