Priestly Benediction: Video
The priestly benediction is found in the Torah (Numbers 6:24-26) and consists of three blessings, which is why it's sometimes known as the "threefold blessing." On holidays like Rosh Hashanah and Yom Kippur, members of synagogues who are part of the priestly class (cohenim) recite the blessing for the congregation, often with their prayer shawls (tallitot) over their heads and their hands spread in the ancient sign of the Jewish priests (made famous by Spock on Star Trek).
Some synagogues have the custom of saying the blessing each Shabbat; it may be led by clergy, by a member of the congregation, or collectively. Some congregations say "amen" after each line, as is customary after most blessings, while others say the traditional response instead, "kein y'hee ra-tzon."
At a bar or bat mitzvah, some synagogues have the custom of directing the blessing at the child. Sometimes, a member of the clergy will place their hands on the bat or bar mitzvah's head or shoulder sand say the blessing to them, adding a few personal words as well. In other congregations, parents are invited to say the blessing to their child. When the blessing is said quietly or more privately, to the bar or bat mitzvah child, the congregation typically does not say "amen" or "kein y'hee ra-tzon" at the end of each line (usually because they are out of earshot).
Here's an example of a rabbi giving a more public blessing to a bar mitzvah boy, with the synagogue's many clergy (rabbis and cantors) joining in the for the priestly benediction, in both Hebrew and English:
For the words to this blessing in English, Hebrew and transliteration, as well as an audio file to help you learn the pronunciation of the Hebrew, please visit our Priestly Benediction resource.
Hebrew for "my master," the term refers to a spiritual leader and teacher of Torah. Often, but not always, a rabbi is the leader of a synagogue congregation. The first five books of the Hebrew Bible (Genesis, Exodus, Leviticus, Numbers, Deuteronomy), or the scroll that contains them.