Torah Shebaal Peh, the Torah of the mouth refers to the Oral Law or the Talmud and its commentaries. Though easily available today in written, digital or online form, oral face-to-face teaching is still essential to its transmission.
It is a cliche to call Jews the People of the Book but People of the Mouth is probably more accurate.
The Oral Law makes specific the laws of the Torah. For example, the Torah tells us to honour our parents, but the Oral Torah tells us what that means in everyday speech and actions as well as how to negotiate the limits of the commandment, for example in a case of an abusive parent.
In the third century, Rabbi Judah Hanassi realised that Judaism was facing a crisis: the chain of transmission from parent to child was breaking and Torah Shebaal Peh was at risk of fading into oblivion. He resolved to write it down and divided it into the six orders of the Mishnah. His genius was to compose a written text that, in the words of Rabbi Hutner, would not supplant the tradition of mouth to ear.
It may be possible to study the Bible on ones own, with a few guide-books to help you along. That is not the case with Torah Shebaal Peh; its suggestive gaps and laconic phrasing make learning from the mouth of a living teacher indispensable.