The five Force Priestesses were entities that made their home on the Wellspring of Life, the origin point of the midi-chlorians.
These priestesses embodied the relationship between the Living Force, as experienced by sentient beings, and the Cosmic Force, as it relates to the universe's grand design. Residing within the Wellspring of Life, these five beings possessed the ability to shape-shift, altering their forms to appear as Serenity, Joy, Anger, Confusion, and Sadness.
Yoda, a Jedi Master, encountered the Force Priestesses during the Clone Wars. They possessed the knowledge required to transform into a Force spirit.
Despite implications from episodes within The Lost Missions of Star Wars: The Clone Wars suggesting that Qui-Gon Jinn gained knowledge from the Force Priestesses and subsequently guided Kenobi to them, official canon sources, including the official Databank and the now-defunct Encyclopedia on StarWars.com, continue to state that Jinn learned from a "Shaman of the Whills." The Ancient Order of the Whills was initially mentioned in the novelization of the original Star Wars film.
In an interview, Dave Filoni proposed that the Force Priestesses might be a singular entity that evolved into five over an extensive period, explaining, "The way that I reconciled that being is actually one being. It's one ancient being separated over a time that for our perception to be able to see her, she is these many different iconic things presented to us. But she died a long, long, long time ago, she is conscious in the Force, and she has a limited ability to manifest."