According to Exodus 12, lamb’s blood was placed on the Israelites’ doorposts and lintels at Passover.
Each Israelite household was instructed to take a lamb, slaughter it at twilight, and mark the two doorposts and the lintel above the entrance with its blood. The sign identified the houses whose occupants followed the Passover instructions. The account connects this marking with the final plague, during which Egypt’s firstborn died.
The lamb was eaten as a meal with unleavened bread and bitter herbs. The household was told to eat it in haste, with sandals on and staff in hand, ready to leave Egypt. This meal became the foundation of the Israelite Passover observance.
A common mix-up is to say that the blood was painted on the floor or spread over the entire doorway. Exodus specifically names the doorposts and the lintel. The account also distinguishes the lamb’s blood from the unleavened bread eaten during the same ritual.