Found in Numbers 6:24-26, the Priestly Blessing (בִּרְכַּת כֹּהֲנִים) is an invocation by the Almighty for the Priests (the Kohanim) to bless the people of Israel, with a prescribed formula. Verse 27 then says that God will follow through.
This became part of the procedure at the Temple. Each morning and evening the priests would stand on a platform called the dukhan (from which comes the Yiddish verb dukhenen, to deliver the Priestly Blessing) and raise their hands. The community would stand on the stairs beneath, and the priests would say the blessing. There is an alternative name for the Priestly Blessing in some circles, the nesi’at kappayim, the raising of the hands. (More about the dukhening at https://bethshalompgh.org/what-are-those-hands-above-the-ark-doing-originally-published-april-27-28-2018/.)
“The Lord bless you and keep you; the Lord cause His face to shine upon you and be gracious to you; the Lord lift up His countenance upon you and grant you peace,” is the literal translation. Some say that being so literal obscures the literary reference to kings and God granting access as a king might show favor and even companionship to a petulant individual. Thus the more modern translation is “The Lord bless you and keep you; the Lord deal kindly and graciously with you; the Lord bestow the Lord’s favor upon you and grant you the Lord’s friendship.”
During the time of the Second Temple, priests would travel to synagogues to deliver the blessing. It is today a relic of Kohanic ritual. It is found near the end of the Amidah. There are many rituals which have derived from this Blessing. In some synagogues (generally Orthodox), anyone who is a Kohen stands to pronounce the blessing. (There must be a minyan.) If all who are worshipping are Kohanim, some may take the lead over the others. Typically, Israeli practice is to do this every day, but in the Diaspora the practice is of dukhening is much less, and varies, often only done on festivals. Some respond “amen” to each of the statements when pronounced by a Kohen, and when chanted by a hazzan they respond “so may it be Your will.” In Reform congregations, which do not generally grant priestly privilege, the rabbi reads the Blessing. There are very few Conservative congregations practicing formal dukhening by Kohanim.
The Priestly Blessing also is used to bless children, at weddings, and for other special occasions, often with hands spread in the priestly manner. The structure of the blessing is three words, five words, seven words, and God’s name is the second word in each. There are eleven unique words in all, said to be a magic number. There have been those who say that this blessing even can neutralize bad dreams.