Ash Wednesday is the first day of Lent, where we are marked with ashes. The ashes on our forehead remind us of the human condition: Remember that you are dust, and to dust you shall return.
The Church asks us to increase our prayer, fasting and almsgiving. True fasting and prayer should make us more aware that nothing on earth can ultimately satisfy us. We are, as St. Augustine famously wrote, made for God and so our hearts are restless until they rest in Him.