Albert Barnes Commentary Leviticus 14:4

"then shall the priest command to take for him that is to be cleansed two living clean birds, and cedar wood, and scarlet, and hyssop:" — Leviticus 14:4 (ASV)

These birds were provided by the priest for the man. They were not, like the offerings for the altar, brought by the man himself (compare Leviticus 14:4 with Leviticus 14:10); they were not presented nor brought near the sanctuary, nor was any portion of them offered on the altar.

Cedar wood, and scarlet, and hyssop — These three substances were used as the common materials in rites of purification (Numbers 19:8; Psalms 51:7; Hebrews 9:19):

• The cedar, or juniper, whose resin or turpentine was a preservative against decay and was used in medicines for elephantiasis and other skin diseases.
• The scarlet, a tongue or band of twice-dyed scarlet wool, with which the living bird, the hyssop, and the cedar wood were tied together when they were dipped into the blood and water; the color expressed the rosiness associated with health and vital energy.
• The hyssop , probably the Caper plant, whose cleansing virtues as a medicine and use in the treatment of ulcers and diseases of the skin related to leprosy were known to the ancients.

It has been conjectured that the scarlet band was used to tie the hyssop to the cedar, creating a sort of brush convenient for sprinkling.