Al Chet
Al Chet, "On Sin," is a confessional prayer said on Yom Kippur that lists 44 different kinds of transgressions we may have committed over the past year.

Apples and Honey
Classic Ashkenazi (Eastern European) Rosh Hashanah food. Both symbolize our wishes for the New Year we're beginning. The apples for a round and full year and honey sweetness. Some substitute date honey (silan) for a vegan alternative. 

Ashamnu
Part of the Viddui services of Yom Kippur, Ashamnu, "We Have Sinned," is an acrostic prayer that mentions different sins we may have committed. It is recited in first-person plural ("we") to indicate that our fates are intertwined on this day.

Atonement
In Judaism, atonement is divided into two categories depending on whether a sin is against other human beings or against God. While God's mercy is seen as limitless, we are required to seek forgiveness three times from other people but no more, should they still refuse to forgive us.

Book of Life
Jewish tradition speaks of God inscribing the names of those God judges, either in The Book of Life or in the Book of Death, each year at the High Holidays. This concept lends itself to the New Year's greeting Chatima Tova, or "good inscription."

Chag Sameach
Chag Sameach means "Happy Holidays" in Hebrew. It is the generic greeting used for joyous holidays (as opposed to fast days, for example, when we would not say this).

Challah
Challah is the Jewish bread traditionally eaten by Ashkenazi Jews on Sabbath and holidays. It is often made with dough enriched with egg and typically braided, and a circular challah is traditional for Rosh Hashanah.

Chesbon HaNefesh
Literally "accounting of the soul," this concept refers to taking stock of our spiritual lives around the High Holidays. We are called on to reflect upon where we are spiritually, as well as where we'd like to be in the coming year.

Days of Awe
The Days of Awe, or Yamim Nora'im in Hebrew, is a term that refers to Rosh Hashana and Yom Kippur as well as to the 10 Days of Repentance which come between them. The name implies the serious reflection and repentance associated with this time of year.

Kiddush
Kiddush is the blessing we say over wine to sanctify special moments. These include Sabbath and Holiday meals, as well as life cycle events like weddings and b'nai mitzvah.

Kol Nidre
Kol Nidre is the first text recited on the eve of Yom Kippur. Technically not a prayer but a declaration, in it, we ask God to preemptively annul any vows we may make in the coming year, so as not to be unwittingly guilty of the sin of failing to fulfill a vow before God.

Lulav & Etrog
Two of the Four Species central to the holiday of Sukkot, the lulav is a date palm frond and the etrog a citron fruit (reminiscent of a large lemon). Along with the other two species, these are used in ceremonious prayer, waved in all the cardinal directions together with special prayers.

Machzor
A machzor is a special prayer book for the High Holidays. Generally, synagogues will have machzorim (in plural) with prayers and services for both Rosh Hashana and Yom Kippur.

Selichot
Selichot, meaning "forgiveness, refers to special prayers recited around the High Holidays. These prayers ask God to forgive our transgressions over the past year.

Shana Tova
Shana Tova means "good year" in Hebrew. It is the most common greeting around the High Holidays and is equivalent to "Happy New Year" in English.

Shofar
The shofar is a bored out ram's horn that is sounded by blowing through the narrower end, without using a mouthpiece. It is one of the oldest known musical instruments, mentioned numerous times in the Torah, and its resonant sonority is meant to call us to reflection and repentance.

Simanim
Literally "signs," the word simanim in Hebrew refers to foods eaten on Rosh Hashanah which are symbolic for their appearance or taste, or for linguistic reasons pertaining to their names. The simanim include gourds, leeks, beets, and dates.

Sukkah
A sukkah (sukkot in plural) is a temporary shelter built for the holiday of Sukkot and used as a temporary home. With an aim to reconnect us to nature, the sukka is built of materials sourced from nature and has a roof that permits a view of the stars.

Tashlich
Tashlich refers to the practice of throwing bread crumbs into a body of water, such as a lake or stream. This is done during the High Holidays as a representation of casting away our sins.

Tishrei
Tishrei is the first month of the Hebrew calendar. It coincides with September-October and is when Rosh Hashana, Yom Kippur, Sukkot, and Simchat Torah all occur.

Torah
The word Torah refers to the Hebrew Bible. Specifically, it refers to the Pentateuch, or the Five Books of Moses (Genesis, Exodus, Leviticus, Numbers, and Deuteronomy).

Yizkor
Yizkor, from the Hebrew word for "remember,"  is a special memorial prayer in memory of the deceased. It is recited at synagogue four times a year, including on Yom Kippur.