Isis is one of the major goddess of the Egyptian pantheon. She is the sister-wife of Osiris, mother of Horus, sister of Nepthys, and daughter of Shu and Tefnut.
Isis and Osiris are credited with bringing civilization to Egypt. Osiris taught agriculture and government, while Isis taught handicrafts, healing and magic. Together They ruled Egypt well, causing Their brother Set to become jealous. Set eventually tricked Osiris, killing Him and usurping His throne.
Isis collected Osiris' body parts, reanimating Him long enough to conceive Horus. She hid along the banks of the Nile, raising Horus until He was strong enough to defeat His uncle Set, and reclaim the throne of Egypt.
For a more complete version of this long and complex myth, see Myth of Isis and Osiris.
Isis is a strong goddess, known for Her magical skills, gained by healing the great god Ra. She was worshiped from the beginning of Egyptian history until at least the 4th century CE, long after most Egyptian gods were forgotten. She is the ultimate high priestess of Egyptian religion.
The High Priestess card indicates mysteries, intuition and feminine consciousness. Isis points to spiritual influence in the current situation. You need to be aware of what your intuition is telling you. It is also wise to consider mystery teachings and their application to the current situation.
The reversed High Priestess card means that you are satisfied with surface knowledge and are unwilling to explore the situation more deeply. You may also be ignoring your intuition.