"Speak to Aharon and his sons, saying: 'So shall you bless bnai
Yisrael...'" (Bemidbar 6:23)
Every kohein has a mitzvah to bless the Jewish nation, for which Hashem
blesses them in exchange. There are three components to this blessing.
The first passuk, "Yevarechecha," refers to physical needs; the second
passuk, "Ya'er Hashem," refers to Torah wisdom; and the third pasuk, "Yisa
Hashem," refers to peace. In many places in Eretz Yisrael these blessings
are pronounced daily by kohanim, while in chutz la'aretz they are reserved
for yom tov.
The Torah teaches us with Avraham Avinu that whenever anyone blesses a Jew
he receives a blessing from Hashem in return. We can utilize this concept
for our own benefit by blessing all those we interact with (Rav Avigdor
Miller, The Beginning, p. 207).