The Force Priestesses were five beings that resided on the Wellspring of Life, the home of the midi-chlorians.
The priestesses were representative of the connection between the Living Force as experienced by living beings, and the Cosmic Force in its role in the greater scheme of the universe. Existing within the Wellspring of Life, the five were a form of shapeshifter, and would mutate their forms to manifest as Serenity, Joy, Anger, Confusion, and Sadness.
Jedi Master Yoda encountered the Force Priestesses during the Clone Wars. They knew the secret to transforming into a Force spirit.