Disclose means to reveal or expose information that has previously been kept a secret — like a politician might be forced to disclose his finances or former scandals while running for office.
When a politician, corporate executive, or celebrity announces that he or she has something to disclose, the public listens. They know that the information they're about to hear was kept secret for a reason. Something incriminating or juicy is about to be revealed, like a secret affair or plummeting company profits.