This is how you shall bless the children of Israel … (Bamidbar 6:23)
One of the biblical commandments reserved for kohanim (priests) is the mitzvah of nesiat kapayim (lit. lifting of the hands) to bless the congregation. The blessing (known as birkat kohanim, the blessing of the priests) is derived from the three verses which follow (Bamidbar 6:24-26).
The Gemara rules that a male, post-Barmitzvah kohen is obligated to bless the congregation if he is in shul at the time for nesiat kapayim (Sotah 38b). According to the Shulchan Aruch (Code of Jewish Law written in 1563) he is not required to find a community in order to perform the blessing (Orach Chaim 128:2). Rabbi Yisrael Isserlein ben Petachia (d. 1460 ) notes that if a kohen does not want to perform the blessing, he may leave the synagogue and return afterwards (Terumat HaDeshen 2:22). However, Rabbi Yisrael Meir HaKohen Kagen (d. 1933) notes that unless he feels ill or weak, it is improper to avoid doing this mitzvah (Mishna Berurah 128:4).
The Talmud implies that it is prohibited for a non-Kohen to perform the blessing (Ketubot 24b). Rabbi Yaakov Chaim Sofer (d. 1939) rules that the prohibition of reciting the text of the blessing only applies if the non-kohen intends to fulfil the mitzvah of birkat kohanim (Kaf HaChaim 128:8). Rabbi Yoel Sirkes (known as the Bach, d. 1640) indicates that it is forbidden for a non-Kohen to raise his hands in the manner of kohanim (Orach Chaim 128).
The Shulchan Aruch rules that when the blessing is taking place, it is forbidden to gaze at the kohanim in order not to distract them (Orach Chaim 128:23). It is also improper to gaze at the hands of the kohanim as the Divine Presence rests on them. This is why the kohanim cover their heads and hands with a talit (prayer shawl).
Since the destruction of the Temple in 70 CE, kohanim have been unable to perform our sacrificial services. While our daily prayers have replaced the Temple offerings (Hosea 14:3), the blessing given to the community by the kohanim reminds us of their unique role in helping to facilitate our connection with God.