mysterious, secret, abstruse
祕 centers on something hidden from ordinary view: mysterious, secret, or abstruse knowledge that is not openly shared.
祕 is a variant form of 秘. It combines 示 (altar, spiritual) with 必, which likely contributes the sound and the idea of something kept closed or hidden. The exact historical development of the 示 form is uncertain, but it reinforces the sense of sacred or concealed matters.
The left side 示 suggests a sacred altar, and the right side 必 looks like a heart with a slash through it, as if something is sealed inside. Picture a secret ritual hidden behind the altar, mysterious and abstruse.
For ヒ, imagine a hidden key (ヒ) that unlocks the secret: the sound 'he' (ヒ) is the key to the mystery.