Déjà-vu literally means “already seen“. There are moments in life when a person has the impression that what they do, see, hear already happened before. Usually, it’s just an impression.
Scientific explanation for déjà vu: there is a split-second delay in transferring information from one side of the brain to the other. One side of the brain would then get the information twice – once directly, and once from the ‘in charge’ side. So the person would sense that the event had happened before.