The High Priestess is arguably the most difficult of the Major Arcana to qualify with words alone, because so much of her power and ability is veiled in mystery that it is difficult for anyone to fathom it all. Every card in the Tarot speaks differently to everyone, but the Priestess seems to bear the widest range of interpretations, because she speaks directly to the Inner Voice, the unconscious mind. She is the manifestation of the unconscious and the effects of the mysterious in our everyday world. Trying to see how such mysteries work often defeats their purpose, and with this in mind, the High Priestess will be explained in as much detail as possible.
She is, above all else, the base of potential that is the source of the power wielded by the Magician. She is the unlimited potential that allows him to transform and create whatever his Will desires. Understanding this type of balance, as found between potential and creation, masculine and feminine, is the key to unlocking some of the mysteries of the High Priestess. We see the clear progression of the theme of balance; instead of integrating opposites, the Priestess keeps them separate and keeps them in balance nonetheles. She herself is the scales; this symbolism is found in many Tarot decks. Without this balance there can be no power.
The second symbolic motif found in almost all instances of the High Preistess are symbols of the unconscious mind. On the Rider-Waite version this motif is especially prominent, but most decks feature at least some lunar imagery that in turn has ties to the unconscious. Most decks that have the "twin pillars" symbology also depict a veil strung between those pillars; the High Priestess stands between us and that veil as a moderator. Behind the veil lies the powers of the unconscious, which we cannot start to understand but which, through her, we can learn to control. She is the gateway to realms that we may never fully comprehend or master.
While it would be impossible for anyone to learn all of her mysteries and secrets, the High Priestess remains as a guide to those of us willing to venture deep within our minds to discover the true powers hidden deep inside each of us. This is the same power as that depicted on the Magician, but the scope of the Priestess' power is far different. While the Magician focuses his powers outward, to achieve a meaningful effect on the world, the High Priestess shows us that we can also use these powers on an inner level, to enrich and transform ourselves. Such transformations are certainly not as dramatic as the Magician's, but they are almost always more powerful.
The High Priestess represents the mysteries of the unconscious and the Inner Voice, and her appearance is often a sign that your own intuition is trying to send you a message. The unconscious often speaks to us in symbols, so be alert around you for anything that seems out of the ordinary. This said, if you have an important decision to make when the Priestess appears, this is often a sign that the answers will be revealed to you, if you are patient and open to the whispers from within. You simply have to wait and be receptive to inner messages. Her lesson is that everything you need to know already exists within you.
The theme of dualism in the High Preistess cannot be avoided either. She is often a sign of the Shadow, the negative portion of your personality that no one sees, and that you yourself could be unaware of. (In this sense, the term 'negative' does not refer to evil, just the opposite polarity from the positive and expressive part of your personality.) If you accept the Shadow within you, its powers will be open to you if you wish to use them. In most people the Shadow side is the more passive of the two, and the Priestess can therefore advocate a need for passivity in a situation. It is not always necessary to act; sometimes goals can be realized through inaction.