Exodus 12:1-13 New International Version (NIV)
The Passover and the Festival of Unleavened Bread
12 The Lord said to Moses and Aaron in Egypt, 2 “This month is to be for you the first month, the first month of your year. 3 Tell the whole community of Israel that on the tenth day of this month each man is to take a lamb for his family, one for each household. 4 If any household is too small for a whole lamb, they must share one with their nearest neighbor, having taken into account the number of people there are. You are to determine the amount of lamb needed in accordance with what each person will eat. 5 The animals you choose must be year-old males without defect, and you may take them from the sheep or the goats. 6 Take care of them until the fourteenth day of the month, when all the members of the community of Israel must slaughter them at twilight. 7 Then they are to take some of the blood and put it on the sides and tops of the doorframes of the houses where they eat the lambs. 8 That same night they are to eat the meat roasted over the fire, along with bitter herbs, and bread made without yeast. 9 Do not eat the meat raw or boiled in water, but roast it over a fire—with the head, legs and internal organs. 10 Do not leave any of it till morning; if some is left till morning, you must burn it. 11 This is how you are to eat it: with your cloak tucked into your belt, your sandals on your feet and your staff in your hand. Eat it in haste; it is the Lord’s Passover.
12 “On that same night I will pass through Egypt and strike down every firstborn of both people and animals, and I will bring judgment on all the gods of Egypt. I am the Lord. 13 The blood will be a sign for you on the houses where you are, and when I see the blood, I will pass over you. No destructive plague will touch you when I strike Egypt.
For the Israelites to be spared from the plague of death. A lamb with no defects had to be killed, and its blood placed on the doorframes of each home. What was the significance of the lamb? In killing the lamb, the Israelites shed innocent blood. The lamb was a sacrifice. It was a substitute for the person who would have died in the plague. From this point on, the Hebrew people would clearly understand that for them to be spared from death, an innocent life had to be sacrificed in their place.