He brought the blood of the slain bullock and sprinkled it on the mercy seat in the Jewish Tabernacle and later in time in the Temple. By this act of obedience to God’s command, atonement was made for the High Priest’s sins and the sins of the priests.
In similar matter, a goat was taken, slain and its blood was sprinkled on the mercy seat to make an atonement for the people of Israel.
Within the ark, which was in the Tabernacle, was found the ten commandments which God gave as a basic teaching of the law.
When God looked down from Heaven he saw this tablet of stone upon which were engraved the ten commandments. Men and woman have disobeyed God’s commandments. To disobey God is to sin.
But then Aaron brought the blood and sprinkled it on the top of this mercy seat, God saw the covering of the blood of atonement which was done in obedience to His command.
Then, God did not see the broken law and our sins, but He saw the covenant of the Blood the Atonement, and God forgave the sins of the people.
http://www.godandjews.org/yom_kippur_the_atonement_today.htm