In his Introduction to the Mishnah, Maimonides tells us that when Moses went up Mount Sinai to receive the Ten Commandments, God taught him the entire Torah. In addition, God gave Moses orally a detailed explanation of every mitzvah. Moses recorded the laws on a scroll—Torah shebiktav, the Written Torah—which was eternally fixed and unchanging. He then verbally transmitted the explanations of the laws to the people. This vast body of knowledge is known as Torah sheb'al peh, the Oral Torah. It had to be memorized and was passed down from father to son, from master to disciple. Thus, an unbroken chain of tradition came into being, a heritage spanning the generations, a timeless legacy reaching back to Sinai.
For example, the Torah says, “You must live in sukkot [thatched huts] for seven days” (Leviticus 23:42). But it does not mention the specifics of the sukkah, its walls, and the material to be used for its roof. These details were given orally to Moses along with the pertinent scriptural verses.
Another example: The Torah says that the Jews should “make zizith on the corners of their garments” (Numbers 20:38). But how are we to make the zizith? How many threads and how many knots? How should they be attached? The Torah does not say. God gave these details to Moses by word of mouth.
Another example: The Torah says, “You must love your neighbor as you love yourself” (Leviticus 19:18). But what are the ramifications of this commandment? These are discussed in the Talmud.