Being acceptable to God is spoken of as being clean or white. God makes people acceptable by forgiving their sins. (See: [[rc://en/ta/man/translate/figs-metaphor]])
The writer speaks of God as if God were a priest who would sprinkle water on him to make him acceptable to God. AT: "Make me acceptable by sprinkling water on me with hyssop" or "Forgive me for my sins so that I will be acceptable to you" (See: [[rc://en/ta/man/translate/figs-metaphor]])
This is a plant that the priests used to sprinkle water or blood on people or things to make them ceremonially clean, that is, acceptable to God. (See: [[rc://en/ta/man/translate/translate-unknown]])