Actually, it's an idiom, and it means: "an intangible something", as in: "Lindsay Lohan may be a train wreck, but she still has a certain je ne sais quoi that makes her attractive."
A literal translation of "je ne sais quoi" as "I know not what" is like saying "It's raining cats and dogs" means there are cats and dogs falling from the sky.
A literal translation of "je ne sais quoi" as "I know not what" is like saying "It's raining cats and dogs" means there are cats and dogs falling from the sky.