To understand what it means, we need the rest of the phrase. Here are some possibilities depending on the context:
* "Quinze et un" means "fifteen and one" (sixteen).
* "Quinze et demi" means "fifteen and a half".
* "Quinze et quelques" means "fifteen and some" (a bit more than fifteen).
* "Quinze et après" means "fifteen and after" (from fifteen onwards).
Please provide the full phrase so I can give you an accurate English translation.