Ramban

THE ETERNAL BLESS THEE, AND KEEP THEE. By way of the Truth, [the mystic teachings of the Cabala], the meaning thereof is because “the blessing” comes from on High [downward], and the meaning of shmirah [“keeping” — and He ‘keep’ thee] is that you should “keep” it, like Remember [the Sabbath-day], and ‘Shamor’ (Keep) the Sabbath-day. Furthermore, the Great Name will make His countenance that watches over you shine [upon you]. And so did the Rabbis say in the Sifre: 41. “Rabbi Nathan says: [The Eternal make His countenance shine upon thee.] This is a reference to the light of the Divine Presence.” And He be gracious unto thee, that you may find favor in His sight, similar to that which the Rabbis have said [that when G-d completed the creation of the world He said]: “My world! My world! Would that you will find favor in My sight all the time [as at this moment]!” Furthermore, the Eternal lift up His countenance to the heavens above and give thee peace — all in your house. And so the Rabbis said in the Sifre: 42. “[And He give thee peace.] Rabbi Nathan said: This is a reference to the peace of the kingdom of David.” So shall they put My Great Name upon the children of Israel, and I will bless them, for it is I that speaketh, behold, here I am, and peace.