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.
–Rabbi Adin Steinsaltz