The weird feeling you get when you remember a scene or event when you know you're actually experiencing it for the first time. Some people take it as evidence that they've lived a past life, but more than likely, it's just an uncommon but harmless chemical or electrical blip in your brain.
Déjà vu is often a fairly enjoyable thing to experience (especially when you have double-déjà -- the "I've experienced this period of déjà vu at least twice before" event) but it's not anything you want to base life lessons or philosophy on.