It's often used to refer to something that's already been said or written, like:
* "I don't want to repeat what I've already said, so I'll just say 'déjà écrit!'"
* "This plot feels very déjà écrit, I've seen it in other movies."
It can also be used as a literary term to describe a feeling of familiarity with something that's new, like:
* "The story had a strange sense of déjà écrit, even though I knew it was original."
Let me know if you'd like to know more about French phrases!