Kabbalah literally means “receiving”.
In Israel today, the receipt you get when making a purchase is called a kabbalah.
The Bible, or Written Law, is given by God and is available to anyone who can read it.
The Oral Law–which includes the Kabbalah–is received, passed directly from teacher to pupil.
Most of Torah is considered Torah haniglet–revealed Torah, to be studied by all Jews.
Kabbalah, however, was designated chochmah nisteret–hidden wisdom.
In truth, Kabbalah was never literally hidden, but was not widely, or even publicly, studied.
The reason for restricting the study of Kabbalah relates to its subject matter.
–Rabbi Adin Steinsaltz