Numbers 19:8
And he that burneth her shall wash his clothes in water, and bathe his flesh in water, and shall be unclean until the even. And he that burneth her shall wash his clothes in water, and bathe his flesh in water, and shall be unclean until the even.
The verse belongs to the red heifer ceremony that provided purification from ceremonial defilement, especially the defilement caused by contact with death. That defilement represented the pollution of sin, and the ceremony as a whole symbolized cleansing from sin. The sacrifice of the red heifer pointed to Christ's ample offering to redeem and purify sinners and to the cleansing made possible by His blood. Beyond that meaning of the ceremony as a whole, no separate explanation is given for the specific washing required of the one who burned the heifer.
The verse shows that God allowed no impurity of person or clothing in Israel, and anyone personally unclean was shut out of the camp until evening and had to wash himself and his clothing before he could enter. The demand for cleanliness extended to the premises for a great distance around the encampment, lest the Lord pass by and see uncleanness. God requires no less cleanliness of His people now than of ancient Israel, and neglect of cleanliness induces disease, so sickness and premature death do not come without cause.