This commentary is the Oral Tradition, also known as the Oral Law, or the Oral Torah. The Written Bible is completely incomprehensible without the Oral Tradition. To demonstrate, I will cite some examples of Laws from the Written Torah that are completely incomprehensible without knowledge of the Oral Tradition. The Oral Law explains why. It also explains the seemingly odd wording of the commandment.
Most Hebrew words change their meaning when pronounced differently.
These are just a few examples of why the Oral Torah is necessary. And if you consider all that the Torah includes, you will realize that the entire body of Torah, the instructions on how to live our lives, is too vast to be confined to a few small books. The existence of the oral tradition is alluded to in the Written Law in numerous places. Nowhere in the Written Torah is such a manner described. Though the laws of slaughtering cattle are not explained in the Written Torah, they are described in detail in the Oral Law.