וְהָיָה֩ הַדָּ֨ם לָכֶ֜ם לְאֹ֗ת עַ֤ל הַבָּתִּים֙ אֲשֶׁ֣ר אַתֶּ֣ם שָׁ֔ם וְרָאִ֙יתִי֙ אֶת־הַדָּ֔ם וּפָסַחְתִּ֖י עֲלֵכֶ֑ם וְלֹֽא־יִֽהְיֶ֨ה בָכֶ֥ם נֶ֙גֶף֙ לְמַשְׁחִ֔ית בְּהַכֹּתִ֖י בְּאֶ֥רֶץ מִצְרָֽיִם׃
내가 애굽 땅을 칠 때에 그 피가 너희의 거하는 집에 있어서 너희를 위하여 표적이 될지라 내가 피를 볼 때에 너희를 넘어가리니 재앙이 너희에게 내려 멸하지 아니하리라
Rashi on Exodus
והיה הדם לכם לאת AND THE BLOOD SHALL BE UNTO YOU FOR A SIGN — it shall be to you for a sign, and not to others for a sign (Mekhilta d'Rabbi Yishmael 12:13:1). From this we may learn that they put the blood only inside their houses.
Sforno on Exodus
ולא יהיה בכם נגף למשחית, so you will not be struck with a plague as a corollary to the retributive action I am taking against Egypt.
Or HaChaim on Exodus
ולא יהיה בכם נגף למשחית, "so there will be no plague against you to destroy, etc." What is meant is that the destructive force (angel of death) will have no excuse to attack you rather than his other natural targets; he will notice the blood on the doorposts of your homes as if it were the seal of the Almighty and be afraid to approach and inflict harm. This was the reason G'd commanded the Israelites to use the blood as a sign. Should you say that in that case any Egyptian hiding out in the house of an Israelite which bore the sign of that blood could escape the plague, the Torah wrote בכם, the restrictions on the angel of death applied only "to you." The "light" of having performed the commandment illuminated only Israel; we find confirmation of this in Isaiah 60,2: ועליך יזרח ה וכבודו עליך יראה, "but upon you the Lord will shine and His Presence will be seen over you."