Final answer:
The Israelites marked their doors with red paint to signal the Angel of Death to pass over their homes, thus avoiding the death of their firstborn during the final plague in Egypt.
Step-by-step explanation:
To avoid the death of their firstborn during the last and most terrible plague, the Israelites were commanded by Moses to mark their doors with red paint. This sign led the Angel of Death to pass over the marked homes, sparing the children within from the plague. This event is commemorated annually in the Jewish tradition through the Passover seder, a ritual meal that marks the beginning of the holiday of Passover, celebrating the Israelites' miraculous salvation and liberation from slavery in Egypt.