He is to sprinkle some of the blood on it with his finger seven times to cleanse it and consecrate it from the uncleanness of the Israelites.
When the priest had come out from the sprinkling the blood of the bullock before the mercy-seat, 1. He must next kill the goat which was the sin-offering for the people (Lev 16:15) and go the third time into the holy of holies, to sprinkle the blood of the goat, as he had done that of the bullock; and thus he was to make atonement...
Commenting on Leviticus 16:15-19
And he shall sprinkle of the blood upon it with his finger seven times,.... This was done with his right finger, or forefinger, as the Targum of Jonathan, and seven times, to denote the perfect cleansing of the altar with it.
Lev 16:18-19 After he had made atonement for the dwelling, Aaron was to expiate the altar in the court, by first of all putting some of the blood of the bullock and he-goat upon the horns of the altar, and then sprinkling it seven times with his finger, and thus cleansing and sanctifying it from the uncleannesses of the children of Israel.
Commenting on Leviticus 16:18-19