In French, déjà vu means "already seen"—the perfect description for this particular sensation. Maybe you're in the middle of a conversation with a friend and suddenly feel like it's familiar or perhaps you visit a place for the first time and find yourself feeling like you've been there before. Whatever the details of the weird, eerie moment may be, you'll know it when you feel it.
The most common theories may answer the (kind of unanswerable) question: What is déjà vu? You might find significance in these moments, if you're open to it.