When the LORD passes through to strike down the Egyptians, He will see the blood on the top and sides of the doorframe and will pass over that doorway; so He will not allow the destroyer to enter your houses and strike you down.
Jewish Publication Society of America Version (JPS1917) - Exodus 12:23 >For the LORD will pass through to smite the Egyptians; and when He seeth the blood upon the lintel, and on the two side-posts, the LORD will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you.