Charles Ellicott Commentary Exodus 12:7

"And they shall take of the blood, and put it on the two side-posts and on the lintel, upon the houses wherein they shall eat it." — Exodus 12:7 (ASV)

Strike it. —With a bunch of hyssop. (See Exodus 12:22.)

The two side posts and on the upper door post. —The idea seems to have been that the destroying influence, whatever it was, would enter the house by the door. The sight of the bloody stains above the door and on either side would prevent it from entering.

The word translated “upper door post” appears to be derived from shâcaph, “to look out,” and to signify properly the latticed window above the door, through which people observed those who knocked before admitting them. Such windows are frequently represented in the early Egyptian monuments.

The blood thus rendered conspicuous would show that atonement had been made for the house, i.e., for its inmates.