Cleanse me with hyssop, and I will be clean;
wash me, and I will be whiter than snow.
David stands as a king before God but is ceremonially unclean and unfit for worship at the tabernacle.
He asks for hyssop, a plant used to purify lepers.
David stands like a leper before God and asks God to "wash" him. The word isn't spiritual, it's practical.
It describes the physical process of washing a garment by treading, kneading, and beating it.
The point: There's a cost to cleanliness