What did the Israelites put on their doorposts?

Answer: Lamb's blood

They put the lamb's blood on the doorframes; when the LORD saw the blood, He would pass over their houses.

“They are to take some of the blood and put it on the sides and tops of the doorframes of the houses where they eat the lambs. The blood on the houses where you are staying will be a sign; when I see the blood, I will pass over you. No plague will fall on you to destroy you when I strike the land of Egypt.”— Exodus 12:7, 12:13 (BSB)