sources: mondly, Lawless French, regardsurlefrançais.com
Literal translation: "Impossible isn't French"
Français here doesn't refer to the French people (as one might think) but rather to the French language. So the translation should rather be: "Impossible isn't a French word"
It can be used in the sense of: "nothing is impossibe"
The site regardsurlefrançais.com gives the explanation:
cette expressions veut dire: Ce n'est pas si difficile que ça. Ce n'est pas grand-chose