This is one of those questions you need to be very careful answering in case you fall into traps: revealing insanely personal things the interviewer doesn't want to know, or things most people would dislike about you that you don't share with your friends, or things they are not allowed to consider in hiring like your religion, sexuality, and so on. If you've considered it in advance, you should be able to make a story that is true and positive.
First and foremost this needs to be true. Don't think about what you can invent. Second, it needs to be a good thing, like being smart or a hard worker or very honest or patient or whatever. Third, there needs to be a reason your friends don't know. For example, perhaps most of your friends are not in your industry and don't know how crazy good you are at something that's very relevant to the job you're applying to. Or they don't know that you dream of a particular technical achievement (writing a book, delivering the keynote at a particular conference, writing a paper that becomes part of the standard for your language, etc.) Or your friends are in your industry and they just think you're talented and don't know that you put in an hour a night watching conference talks which is why you know things they don't.
This is a handy ability to have - tell us something about you - so practicing it now won't be a waste of time. The wrinkle that your friends don't know it makes it a little harder, but you should be able to, in the more relaxed time you know have, come up with something. Practice saying it in a reasonable number of sentences: 3 or 4 is probably right.
