It is a positive biblical commandment for the Kohanim to bless the nation of Israel, as it is written (Numbers 6:22-26), “Hashem spoke to Moshe, telling him to speak to Aharon and his sons saying, ‘This is how you must bless the Israelites. Say to them: May God bless you and keep watch over you. May God illuminate His face towards you and be gracious to you. May God bestow favor upon you and grant you peace.’”
This mitzvah is performed daily. Every Kohen who is asked to ascend the duchan (platform) to bless Israel but refuses, even though in principle he only nullifies one mitzvah, is considered to have nullified three biblical commandments. This is because the wording used in the verses implies that Hashem longs to bless Israel. Therefore, a language of command is employed three times to tell the Kohanim to bless Israel, as it is written, “This is how you must bless,” “Say to them,” and “Place My Name.” Hence, a Kohen’s refusal to fulfill the will of the Creator and bless Israel is considered a nullification of three mitzvot from the Torah (Sotah 38b; Rambam Tefillah 15:12).
Even if a Kohen who already blessed Israel that day was summoned by another minyan to bless the nation, it is a mitzvah for him to ascend and bless again. However, if he refrains, he does not nullify a biblical commandment (Shulchan Aruch 128:3).
The Sefer Charedim (12:18) writes an important novel insight, that not only the Kohanim fulfill a mitzvah from the Torah by blessing the nation, but the Israelites who stand before them in silence with kavanah and respond Amen to their blessing also participate in the fulfillment of this biblical commandment.
In Birkat Kohanim, we learn to focus on the fundamental fact that HaKadosh Baruch Hu is the One who bestows blessing upon us, and our standing each day in readiness for Birkat Kohanim instills this belief in our hearts (Guide to the Perplexed, part 3, chapter 44; HaAkedah, Sha’ar 74). The more we are aware of the fact that HaKadosh Baruch Hu is the One who blesses the nation of Israel with love, the more we will be open and prepared to accept the blessing (see Sefer HaChinuch 378). Free will is the fundamental component of the world, and therefore the blessing that HaKadosh Baruch Hu bestows upon us is linked to our own efforts. Or, as expressed by the Kabbalah, the awakening of our will to accept His blessing awakens the Supreme Will to bestow blessing upon Israel in abundance. By fulfilling the mitzvah of Birkat Kohanim, the nation of Israel expresses its will to accept the Divine Influence (shefa) and draws Hashem’s blessing into the world.