The vast majority of words in the Hebrew language can be boiled down to a three-consonant root word that contains the essence of the word's meaning. For example, the first word of the Torah is "bereishit", meaning in the beginning. The root is Resh-Alef-Shin, which means head or first. (See Hebrew Alphabet to learn the letters). It is the same root as the "Rosh" in "Rosh Hashanah" (first of the year, i.e., Jewish New Year).
There are surprisingly few root words in biblical Hebrew, but we get a lot of mileage out of the ones we have. For example, from the root word Qof-Dalet-Shin, meaning holy, sacred, or sanctified, we get kedushah (holiness), kiddush (a prayer over wine sanctifying the Sabbath or a holiday), Kaddish (an important prayer commonly thought of as a mourning prayer), aron kodesh (holy cabinet - the place in synagogue where the Torah scrolls are kept), and kiddushin (betrothal).
Less obviously, from the root Samech-Dalet-Resh, meaning order, we get siddur (the daily prayer book, which sets for the order of prayers), seder (the Passover family ritual, which must be performed in a specified order) and sidrah (the weekly Torah reading, also called a parashah).
A substantial amount of rabbinical interpretation of the Bible is derived from the relation between root words. For example, the rabbis concluded that God created women with greater intuition and understanding than men, because man was "formed" (yeetzer, Genesis 2,7) while woman was "built" (yeeben, Genesis 2,22; our JPS Bible translates "made He a woman" here). The root of "built", Bet-Nun-Heh, is very similar to the word "binah" (Bet-Yod-Nun-Heh), meaning understanding, insight, or intuition.
If you are interested in Hebrew root words, a good book to look at is Edith Samuel's Your Jewish Lexicon, which looks at a lot of important Jewish concepts and idioms through their root words.
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