It is used to describe someone who is:
* Scared or terrified: The shock or fear causes their face to lose all color, making them appear pale like a ghost.
* Sick or unwell: Paleness can be a sign of illness, especially if someone is pale and has other symptoms like fatigue or weakness.
* Exhausted: Extreme tiredness can also cause a person to appear pale.
Example:
* "After the accident, he was as white as a ghost."
* "She looked as white as a ghost after she heard the news."
The idiom emphasizes the extreme paleness of someone's face, highlighting the emotional or physical state causing it.