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.
KJV
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].
ASV
For Jehovah will pass through to smite the Egyptians; and when he seeth the blood upon the lintel, and on the two side-posts, Jehovah will pass over the door, and will not suffer the destroyer to come in unto your houses to smite you.