I've read many sources that interpret this card using Judaic and Egyptian mysticism. The pillars flanking The High Priestess for example are supposed to represent Jachin and Boaz of Solomon's temple, and she is clearly holding the Torah in her hands. As I don't follow these spiritual practices I have developed my own interpretations from the symbols on the card.
The High Priestess is partner to two other cards in the deck. The Magician who we've already met and The Empress. The Priestess balances both of these cards. While the Magician represents active physical power, The High Priestess represents inactive, mental power. The High Priestess is the impassive and emotionless representation of the sacred feminine in contradiction with The Empress's bubbly warmth. She is the master of platonic relationships while her sister The Empress is the archetypal mother and wife.
1 We see a veil of pomegranates behind The High Priestess. The Pomegranates represent the fertile ground of possibility. Don't be mistaken though, the fertility is not sexual but personal. It is the ground upon which we realize our full potential.
2 Upon The High Priestess's head we see a crown. Some see this as the crown of Hathor, the ancient Egyptian bovine fertility Goddess. It is meant to look like the horns of a cow with a solar disk resting between them. When I see this I see the symbol of the triple Goddess. Both interpretations however further symbolize the fact that she is meant as a feminine archetype.
3 Caught up in her gown we see a crescent moon. The crescent moon symbolizes mystery because it is not fully visible the way the full moon is. Depending on the answers you're seeking and her placement in a reading The High Priestess can represent the unconscious mind. She can also represent all of the things the unconscious mind is capable of. Perception, foresight, and intuition.
4 The Torah she holds might symbolize ancient wisdom and The High Priestess's keen penetrating mind.
5 The cross in the center of her chest is a controversial symbol. While many may see it as a biblical representation of crucifixion. However, since most of this cards symbolism is rooted in Judaic and Egyptian mysticism, and neither the Jewish people or ancient Egyptians worshipped Christ it is much more likely meant to be seen with a broader definition. It can be looked at as representing the four elements, eternal life, health, and union. When seen in this light it represents The High Priestess's self-reliance. She has all of the mysteries of the universe locked within her being.
6 The pillars on either side of The High Priestess aren't just marked with a B and a J, they are black and white. The opposing pillars represent her balance, serenity, objectivity, and sound judgment.
Key Words: Inactive, Impassive, Emotionless, Platonic Relationships, Potential, Mystery, The Unconscious, Perception, Foresight, Intuition, Wisdom, Penetration, Self-Reliance, Serenity, Objectivity, Sound Judgment.
When doing inverse readings: The opposite of The High Priestess is someone who rules from a place of passion. They have poor judgment and aren't willing to dig for truth. They're someone who is happy being ignorant and accepts the first explanation they are given for anything.