In Exodus, lamb's blood was placed on Israelite doorposts so the final plague would pass over their homes.
Before the tenth plague, each Israelite household is instructed to select a lamb, slaughter it at twilight, and put some of its blood on the two doorposts and the lintel. The household is then to eat the roasted lamb with unleavened bread and bitter herbs. Exodus says that when the Lord sees the blood, the plague will pass over that house.
This ritual becomes the foundation of Passover, the festival commemorating Israel's deliverance from Egypt. The meal is also called the Passover sacrifice in the biblical instructions. The haste of departure explains why unleavened bread is central: the Israelites leave before dough can rise.
The blood is not described as red paint, oil, or water. A later instruction makes clear that the rite is a memorial to be observed by future generations. Passover's meaning and practices develop across the Hebrew Bible and later Jewish tradition, but the Exodus doorpost ritual is its narrative origin.