In Exodus 12 we read the account of the first Passover. The people of Israel were slaves in Egypt and God put forth His plan to rescue them my killing the first born in all of Egypt. The Israelites were to take a year old male lamb without blemish, keep it in the house with the family 14 days, kill it and put its blood on the door post of the house. The blood would serve has a covering and protect their firstborn from death.
God gave His people a covering, and that covering required blood, the blood of a perfect lamb. When the angel of death saw the blood on the door post he passed over their home, keeping death away. The death of the lamb spared the life of the first born. "A life for a life" as the saying goes.
This was the first Passover, and we remember it today.