The Oral Law is a legal commentary on the Torah, explaining how its commandments are to be carried out. Common sense suggests that some sort of oral tradition was always needed to accompany the Written Law, because the Torah alone, even with its 613 commandments, is an insufficient guide to Jewish life. For example, the fourth of the Ten Commandments, ordains, "Remember the Sabbath day to make it holy" (Exodus 20:8). From the Sabbath's inclusion in the Ten Commandments, it is clear that the Torah regards it as an important holiday. Yet when one looks for the specific biblical laws regulating how to observe the day, one finds only injunctions against lighting a fire, going away from one's dwelling, cutting down a tree, plowing and harvesting. Would merely refraining from these few activities fulfill the biblical command to make the Sabbath holy? Indeed, the Sabbath rituals that are most commonly associated with holiness-lighting of candles, reciting the kiddush, and the reading of the weekly Torah portion are found not in the Torah, but in the Oral Law. Without an oral tradition, some of the Torah's laws would be incomprehensible. In the Shema's first paragraph, the Bible instructs: "And these words which I command you this day shall be upon your heart. And you shall teach them diligently to your children, and you shall talk of them when you sit in your house, when you walk on the road, when you lie down and when you rise up. And you shall bind them for a sign upon your hand, and they shall be for frontlets between your eyes." "Bind them for a sign upon your hand," the last verse instructs. Bind what? The Torah doesn't say. "And they shall be for frontlets between your eyes." What are frontlets? The Hebrew word for frontlets, totafot is used three times in the Torah — always in this context (Exodus 13:16; Deuteronomy 6:8, 11:18) — and is as obscure as is the English. Only in the Oral Law do we learn that what a Jewish male should bind upon his hand and between his eyes are tefillin ( phylacteries).
Joseph Telushkin, "Jewish Literacy: The Most Important Things to Know About the Jewish Religion, Its People and Its History". NY: William Morrow and Co., 1991; republished in “The Oral Law Talmud and Mishna”, Jewish Virtual Library.