The moon has a long history of association with the feminine, creative principle. Its meaning here seems pretty clear, but what's interesting to me is the fact that it's at her feet. This says to me she's on a higher plane than it is.
The High Priestess is the source, the seed of life. The theme of female fruition can be seen in the Date Palms and Pomegranates on the veil behind her. The words of the Tora she holds in her lap are the seeds that lead to immortality. She is robed in the essence of life, which flows from her and turns to water as it reaches the material plane, by way of the moon. You can see the water flowing through many of the other cards in the major arcana. She is Nature in the abstract sense, the invisible, yet ever-present, Universal Mother. The Empress is her daughter.
fools fool