The church service marks the beginning of the Season of Lent, a time of spiritual warfare, of fasting, penitence, and preparation for the Easter feast. Beginning in the tenth century ashes were imposed on this day upon penitent sinners in preparation for their restoration to full communion with the Church.
The season of Lent lasts for forty days (not including Sundays). It is a time when Christians reflect and prepare for the celebrations of Easter. Some people fast, eat frugally or give up treats following the example of Jesus, who fasted for forty days in the wilderness.
Remember that you are dust, and to dust you shall return.