In the Reclaiming strand of the tradition in which I was initiated, the word "priestess" (which is used for either gender) is also used as a verb; to priestess. Anybody can priestess, not just bona fide initiates with years of intensive training under their belt. This is both a strength and a weakness of Reclaiming, but that's a subject for another post!
But what does "to priestess" actually mean? In the group rituals I have participated in over the past few years, several people have exemplified the qualities necessary for an effective priestess. I think back to my first Witch Camp, with Dawn Isadora talking in ritual about the Goddess in an intimate way, simply referring to Her as "She". My second Witch Camp had four amazing priestesses aspecting the Star Goddess and opening up the vortexes of power.
Then, there is the daily, mundane aspect of priestessing which I am currently involved in. Me and one of my initiators are team-teaching three students via distance teaching. Part of this is agreeing to meet for lesson planning and other details which aren't as glamorous as aspecting in a huge public ritual.
Nevertheless, this is all part of service, which is essential to good priestessing.
Potentially the most important role of the priestess is the ability to open the gates, to provide a space for others to experience the magical, the numinous, and the divine.
Priestesses are the holders of sacred space.
(As an aside, I also believe that when two (or more!) people have sex, they are priestessing each other - they open the gates for the other lover to experience the deep connection that sex can provide.)