God tells them to take a lamb and place its blood on their doorposts, to enable the tenth plague to pass by their houses and affect only the homes of the oppressors.
The lamb is chosen because it is a sacred animal to the Egyptians, and carrying out the Divine command is an act of defiance of Egypt and its ways. The Midrash puts in the mouth of God the warning: “By your lives, you will not depart from here until you boldly stand up to the Egyptians.”
Slaves, prisoners and the downtrodden in every age often get used to their degraded situation. They resent their captors or oppressors, but they learn to live with the degradation. Their songs of yearning for freedom become fainter and fainter.
The message to them is, “Keep your nerve, retain your courage, and earn your redemption!”