Each day, the Kohanim deliver the "Birkat Kohanim" (priestly blessing) to the congregation. This Beracha contains fifteen words, and the holy books teach that the first fourteen words correspond to the fourteen joints on the human hand. Each of a person’s four large fingers contains three joints, and the thumb has two joints, for a total of fourteen. (This is why the Hebrew word for hand is "Yad," which has the numerical value of fourteen.) The fifteenth word of the blessing, "Shalom" ("peace"), corresponds to the palm of the hand. Peace is akin to a palm because it is, as the Mishna describes it, the "receptacle" that contains peace. Without the palm, the hand is unable to hold anything; whatever one receives would slip through the fingers. Peace corresponds to the palm because it is a prerequisite for receiving God’s material blessings.
Incidentally, this is why it is common across much of the world to express friendship, camaraderie, and cordiality with a handshake. The two people connect their palms to one another and share greetings. The palm is the symbol of peace, and the joining of palms thus represents a peaceful, congenial relationship.
Peace is also necessary as a means of eliminating the harmful spiritual forces that interfere with the "Shefa" (bounty) that comes down from heavens. Often, the blessing of Parnasa (livelihood) will descend from the heavens, but on its way down it is blocked by destructive angels, the angels that prosecute against us, and other harmful spiritual forces. The presence of peace clears the air, so-to-speak, allowing the blessing to descend from the heavens without interference.
These spiritual forces also threaten to sabotage our prayers. Our prayers are ineffective if they do not reach their intended destination – the heavenly throne. Needless to say, there is quite a distance that the Tefilot must travel from our mouths to God’s throne in the highest spheres of the heavens. When a guest leaves his host’s home, there is a Misva for the host to escort him ("Levaya") out; rather than just waive and say "Good bye," he should walk four steps with the guest outside, and this has the effect of protecting the guest during his trip his home. All the more so, our prayers, which must travel a vast distance to their destination, and are threatened by untold numbers of harmful forces, require an "escort" for protection.
The holy books say that the "escort" of our prayers is Birkat Kohanim, which we recite immediately after Birkat Ha’Torah right at the beginning of the morning prayer services. These verses were not chosen randomly. Rather, they were chosen because they serve to "escort" our prayers along their journey to the heavens. Indeed, the Zohar writes that there are four divine Names embedded within the text of Birkat Kohanim, and they achieve four different goals: the acceptance of our prayers, Parnasa, adequate clothing, and miracles.
The text of Birkat Kohanim alludes to its function as an "escort" to our prayers. Each of the three verses of Birkat Kohanim begins with the letter "Yod" ("Yevarechecha," "Ya’er," "Yisa"). The numerical value of the letter "Yod" is ten, and thus the combined numerical value of these three letters is thirty, which is represented by the letter "Lamed." There are six blessings in Birkat Kohanim ("Yevarechecha," "Ve’yishmerecha," "Ya’er," "Viy’huneka," "Yisa," "Ve’yasem Lecha Shalom"), and the number six is represented by the letter "Vav." The fifteen words of Birkat Kohanim are represented by the letters "Yod"-"Heh." These four letters ("Lamed," "Vav," "Yod," "Heh") spell the word "Levaya" – "escort" – alluding to this special power of Birkat Kohanim to escort and protect our prayers as they make their way to the heavens.
The final word of Birkat Kohanim, as mentioned, is "Shalom," because that is the key that opens the doors for the "Shefa" to rain down upon us. Birkat Kohanim is effective in bringing our prayers to the heavens, but peace is indispensable for allowing the blessings to come down from the heavens to us. The priestly blessing therefore concludes with this word, to emphasize the critical importance of "Shalom" in enabling our prayers to have the desired effect of bringing the heavenly blessings down to earth.