Numbers 19:9
And a man that is clean shall gather up the ashes of the heifer, and lay them up without the camp in a clean place, and it shall be kept for the congregation of the children of Israel for a water of separation: it is a purification for sin.
A man who was clean, uncontaminated by contact with the dead, gathered up the heifer's ashes. He laid them up outside the camp in a clean place. They were kept there for the congregation of Israel as a water of separation. With water they were used in a repeated ceremony to purify ceremonial defilement.
The rite was a purification for sin. It impressed upon Israel the connection between death and sin. The ashes themselves represent a whole and ample sacrifice. That wholeness and ampleness point to the completeness of Christ's offering.
His complete offering is His spotless righteousness and His shed blood. That blood and righteousness cleanse from sin. The cleansing ceremony points to His work to cleanse the sanctuary. That same work reconciles people to God.