Yes, that’s French! A lot of French expressions are used in English actually!
What it means:
“Je ne sais quoi” means that something is good but for a reason that is difficult to explain.
When to use it:
Use it when you can’t quite say what is that little thing that makes it how it is. This expression will also make you sound a little fancy.
Imagine that both Gordon and you have each made a cake. You both followed the instructions correctly but for some reason Gordon’s is better. You could say that his cake has a je ne sais quoi.
Other Interesting Idioms in J:
Just what the doctor ordered – It means that it is just what you need.
Justice is blind – It means that justice is impartial.
Keep on learning!