1. God instructed the Israelites to sacrifice a lamb on the 14th day of the first month and put its blood on the doorposts of their houses.
2. When God struck down the firstborn of Egypt, He would "pass over" any house with lamb's blood on the doorposts. No plague would touch those houses.
3. The blood was a sign for God to pass over the Israelites and spare them from the destructive plague that struck down the Egyptians.