The High Priestess is the card of knowledge, instinctual, supernatural, secret knowledge. She holds scrolls of arcane information that she might, or might not reveal to you. The moon crown on her head as well as the crescent by her foot indicates her willingness to illuminate what you otherwise might not see, reveal the secrets you need to know in order to make a decision about a problem or a job, an investment, love, career, family, etc.
And, finally, there is, behind her throne, the curtain that leads to the deepest, most esoteric and secret knowledge; the pomegranates that decorate it remind us of Persephone, who was taken down into the land of the dead, ate its fruit, and became the only goddess allowed to travel to and from that strange land. Which indicates that when you get the High Priestess, you're going to be learning some very odd things. Very odd.
As a card, the High Priestess is about knowledge. "I've a new idea," says the querent--thanks to the Magician. Maybe they've realized they want to be an painter or run for office or open their own business. But how do they decide what they want to paint? How do they decide which public office to run for? How do they find out where to start their new business? Knowledge. Insider knowledge from some old expert being the best. The more secrets the querent knows, the easier it is to know what to do with the idea. This is the job of the High Priestess, to offer secret knowledge, like the moon on a dark night, so that the querent can find their path. She sits between the pillars of dark and light, existence and negation, wax and wane. All secret knowledge is hers.
The High Priestess does not seek to dissect. Instead she relies on synthesis, on the bringing together of opposites. She is duality in a singularity. An emotional singularity which can suck you in and from which you may never escape.
She knows all your secrets, you can hide nothing from her. Yet you will never know the secrets she herself protects.