Undisclosed means not made public — the information exists, someone knows it, and they have chosen not to say. It is almost always used in front of a noun, and a small set of nouns at that: an undisclosed sum, an undisclosed amount, an undisclosed fee, an undisclosed location, an undisclosed source, undisclosed reasons. If you learn the phrase "for an undisclosed sum", you have learned most of this word — it is how news reports describe a company or a footballer being sold when nobody will say the price. This is the language of business pages, court reports, and press releases, not of conversation. What makes it useful, and worth noticing, is its neutrality. "Secret" and "hidden" both hint that someone is up to no good; undisclosed simply reports that the number was not given, and often the not-giving was perfectly normal and agreed by both sides. It even appears where a rule requires telling: "undisclosed conflicts of interest" means the person was supposed to disclose and did not — there the blame comes from the situation, not from the word itself. "Withheld" is its close cousin but sounds more deliberate and more suspicious.