You can use the phrase je ne sais quoi when you're describing something that's hard to put into words. If you say something has a certain je ne sais quoi, you'll sound fancy instead of inarticulate.
In French, je ne sais quoi literally means "I don't know what." It's used to capture an indescribable, special distinguishing feature, or to name some unnamable quality. You could say, for example, "Ms. McMane's English class isn't like any other class I've taken — it has a certain je ne sais quoi."