ELI5 Deja Vu - ever remember you forgot something but can't remember what?

What is deja vu? Why does it occur? Explain the weird sensation when you remember that you forgot something, but can't remember what it is that you forgot? What is presque vu? How about jamais vu - often described as the opposite of deja vu?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: nobody_o, jxf, keligrein & solensia

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