Throughout the Old Testament, God has given many examples of what His one and perfect sacrifice would look like. One of these examples was Passover.
When the Jews were slaves in Egypt, God told Moose that He would send the angel of death to kill the first born of every house hold.
The family had to take a perfect lamb with no spots or defects and bring into their home to live with them for a week. Then, they had to kill the lamb and smear the blood on the door posts of their home. When the angel of death came, he would see the blood and pass over the house, spearing the first born.
By the blood of the lamb, death passed.