The High Priestess is the next person the Fool visits on his journey through the Tableau. She is number 2.
You see that she has two pillars and a crown. She is also holding a book (but sometimes it is a scroll), and with a cat at her feet. This is all important because the High Priestess is symbolizing the Tree of Life (which you can see in a diagram at the top of the page). There are three pillars on the tree of life. She sybolizes the third, and you can see the other two. The crown is a part of the tree, that is essentially...God. This isn't often shown in images of the Tree of Life, but it's still there. That book you see? It is a religious scripture, sometimes the Torah, but it can be other things as well. Often times when it is a scroll, it symbolizes the Torah, indeed it oftentimes says Torah.
The High Priestess' job is to tell the Fool that there is a greater mystery...a reason for being...the meaning of life. Her job is not to tell him what the mystery is. Just that there is one.
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.