Fear Projection is the ability to read one's fears and bring them to life. The only known being to possess this power is Barbas, the Demon of Fear. The purpose of this power is to literally scare beings to death by confronting them with their worst fears.
The exact function of this power is somewhat a mystery, as it's effects differ depending on the situation. The fears can either manifest as illusions or the fears can literally come to life. This is evident as the projected fears sometimes leave behind physical evidence, such a projected fire leaving behind burn marks.
Another aspect of this power is to use fears to manipulate and brainwash victims. Barbas once brainwashed Prue Halliwell into thinking her sisters were replaced with demons. Prue then attacked her sisters until they managed to convince her they were themselves.
Being confronted by one's fears can have a paralyzing effect on beings. In the case of witches, they may become unable to access their powers, as these are tied to their emotions. When a victim is scared to death, their hair turns white as a result.