“Kabbalah is the inner, mystical dimension of the Torah, corresponding to the level of sod.
As indicated by its name, which means ‘what has been received’ or ‘tradition,’ Kabbalah is based on traditions received from one’s teachers, who received them, in turn, from their teachers.
Kabbalah is not a separate area of Torah knowledge but rather the hidden, spiritual dimension of the revealed aspects of the Torah.
Whereas the revealed aspects of the Torah, such as halakhah, speak primarily about visible, physical things, Kabbalah speaks directly about spiritual entities.
It speaks of the system of Worlds and sefirot through which God creates, sustains, and directs the universe; and it discusses the interaction between those spiritual entities and the performance of mitzvot in the physical world.
Hence Kabbalah has been called the soul of the Torah.
All Torah study is based on the acceptance of tradition and on the principle that because the Torah is a divine gift, a person must make himself into a proper vessel in order to receive it.
In the study of Kabbalah, however, these approaches are even more important.
Because Kabbalah is the inner spiritual dimension of the Torah, the individual must study it in a way that engages his inner, spiritual dimension.
A person who wishes to study Kabbalah should already have an inner understanding of the ideas, and he must pursue the study of Kabbalah in a spirit of purity and holiness, in order to become a suitable vessel.”
From Opening the Tanya, p. 304, by Rabbi Adin Steinsaltz