They possess magical abilities that greatly exceed those of a modern mage. However, they do not have the same raw ability that a true magician such as Beranabus does. Priestesses while highly respected are also deeply feared. This mostly stems from their reclusive nature and superstition from regular humans
Priestesses draw their power from the Earth and it's flora and fauna. While their male counter-parts, Druids draw their magic from the stars and beyond. Due to one of their main sources of magic being the Sun, Priestesses are stronger during the day, especially when the sky is clear.
Priestesses are exclusively female mainly because of the source of their magic. Men draw power more easily from the stars and beyond while women draw their power more easily from the Earth. However, there doesn't seem to be any discernible difference in strength or potential between the two.
"A priestess needs to soak up the power of the world around her, to draw strength from the land, the wind, the animals and trees"
(Bec - pg 10)
There are only two known Priestesses, all mentioned in book 4 Bec: