My favourite card. Here is everything that I know about her:

Secrets, mystery, the future as yet unrevealed; the woman who interests the Querent, if male; the Querent herself, if female; silence, tenacity; mystery, wisdom, science. Reversed: Passion, moral or physical ardour, conceit, surface knowledge.

The scroll in the High Priestess' hands is inscribed with the word Tora, signifying the Greater Law, the Secret Law and the second sense of the Word. It is partly covered by her mantle, to shew that some things are implied and some spoken.
from Waite's Pictorial Key to the Tarot

She is adorned with the headdress (Waite calls it a diadem) of the Ancient Egyptian god Hathor. However, its symbolism represents the ancient Egyptian godess Isis: the roles of Hathor were eventually assimilated into those of Isis. Isis is a mother god, and is associated with light, life, and the earth. Her ethereal robe, the crescent moon at her feet, and her solar cross suggest maternal aspect:

She is, in fine, the Queen of the borrowed light, but this is the light of all. She is the Moon nourished by the milk of the Supernal Mother.

The scenery around the Priestess is described in the Torah, in 1 Kings 7.13-23:

He (an appointed coppersmith) cast two columns of bronze; one column was 18 cubits high and measured 12 cubits in circumference, and similarly the other column. 16 He made two capitals, cast in bronze, to be set upon the two columns, the height of each of the two capitals being 5 cubits; 17 also nets of meshwork with festoons of chainwork for he capitals that were atop the columns, seven for each of the two capitals. 18 He made the columns so that there were two rows of pomegranates encircling the top of the one network, to cover the capitals that were on the top of the pomegranates; and he did the same for the network on the second capital. 19 The capitals upon the columns of the portico were of lily design, 4 cubits high; 20 so also the capitals upon the two columns extended above and next to the bulge that was beside the network. There were 200 pomegranates in rows around the top of the second capital. 21

He set up the columns at the portico of the great hall; he set up one column on the right and named it Jachin, and he set up the other column on the left and named it Boaz. 22 Upon the top of the columns there was a lily design. Thus the work of the columns was completed. 23
from the JPS (Jewish Publication Society) Tanakh Translation

Behind the High Priestess are two pillars, one black and the other white; they are bridged by a netted tapestry depicting pomegranates and (perhaps) palm fruits. This arrangement is that of Soloman's Temple, the first Temple of the ancient Israelites. The left pillar is Boaz (means strength), the right Jachin or Jakin (establishment/founding). They Jewish Study Bible suggests that they may be read together as may he establish in strength

The arrangement of the pomegranates on the netting behind the High Priestess could be interpreted as the shape of the Tree of Life, the Kabbalistic path to God. Theologically, she is associated with the Kabbalistic Shekinah:

According to Kabalism, there is a Shekinah both above and below. In the superior world it is called Binah, the Supernal Understanding which reflects to the emanations that are beneath. In the lower world it is Malkuth--that world being, for this purpose, understood as a blessed Kingdom that with which it is made blessed being the Indwelling Glory. Mystically speaking, the Shekinah is the Spiritual Bride of the just man, and when he reads the Law she gives the Divine meaning.

The scroll in her hands is inscribed with the word Tora, signifying the Greater Law, the Secret Law and the second sense of the Word.