After the ninth plague, Pharaoh still would not let Moses lead the Israelites out of the slavery of Egypt into the Promised Land. When God spoke to Moses, He prepared him for the tenth and final plague. He told him that every firstborn would die unless each family prepared in a certain way.
Then the Lord God of Israel shared the secret. It was in the blood—the blood of a pure, healthy, and young lamb. This was their door of escape. So Moses told the people to swab blood on the lintels of the doorposts for protection and the Death Angel would pass over… and the firstborn would be spared.
As awesome as this story is, it is just a shadow of good things to come. Later, God provided His only begotten Son, sinless and blameless, as the Passover lamb to die on the cross. That provided protection from eternal death and separation from God.