Déjà vu is an interesting concept. It is when someone experiences a familiarity with an experience that, to their knowledge, they have never had in their lifetime. There is a lot of information about it on the internet, such as this Wikipedia link, but from what little I’ve read, it’s very difficult to prove as it is virtually impossible to duplicate.
I don’t know how to explain how it happens, if that is what you mean. There are are a lot of theories on that. One is about memories that get passed on through DNA. Another is based upon reincarnation.
As to what I think about it, I’m open to the possibility. I’ve experienced something similar to it, although in reverse.