The High Priestess is the symbol of infinitely deep wisdom. She knows and understands everything, in nuances that you can't comprehend. She is the patient teacher, whose vast wisdom encompasses every paradigm you have known, and already understands the things you aren't ready to understand, knows the things you will come to understand, and the things that you will never understand.
High Priestess tells you that it's time to withdraw from involvement, allow events to proceed without intervention. Be calm, and wait patiently. It's a time to study, learn, and contemplate - not to act. It instructs the questioner to watch carefully, look beyond the obvious, and seek what is hidden. Don't seek guidance from others; the insight you need is already within you. Find the serenity in your mind and there you'll find truth.
For me, the idea of "watch but don't act" is what evoked the quarter-note ringing ostinato introduced from the start. The melody is gentle and thoughtful, but deliberately placid, passive. It's not a busy melody, and it isn't one that demands attention; it's a melody that hovers above the scene watching from a discrete distance, observing.
The gentle middle section explores one of the themes, letting go of the ringing tones for a while to wander outside of obvious patterns. While it closely observes the previous section, it searches for insights hidden in its folds, between the lines. After a short journey, we discover the theme's hidden clarity, and then we return to watching, but with a more informed perspective.
The insights gathered in the middle section informs the ending, and the initial themes are revisited again - without significant changes (no action taken), but now informed by the journey that came previously. The piece ends calmly with a delicate departure, not in the manner of a door closing, but more as a boat sailing away toward a horizon.