The Musaf Service--which is held immediately following the Morning Service) consists of the Musaf Amidah, the cantor's repetition of the Amidah, the avodah -- a recounting of the Yom Kippur service in the Holy Temple -- and the priestly blessing.
The priests, or kohanim, direct descendants of Aaron, the High Priest, are commanded to bless the Jewish people with a three-fold blessing. It is customary to perform this duty during the Musaf service on festival days. In preparation for the blessing, the priests remove their shoes and the Levites ritually wash their hands, they then gather at the front or by the eastern wall of the synagogue.
During the blessing, one must not gaze at the Priests directly as the Divine Presence rests upon them. It is customary for men to cover their eyes with their prayer shawls, and for women to gaze into their prayer books.
After the blessing, it is customary to thank the Priests with the words, Yishar Koach.