|Some dare call it|
By definition, disclosure is the sharing of confidential information. It is often used in the context of important corporate or government information, such as company or national secrets and activities. When agreeing to work for some corporations or branches of government, employees may have to sign a non-disclosure agreement or NDA. This is usually so that if they sell, spread, or otherwise leak secrets that could cause the company or government branch to lose money, lose security, or lose confidentiality they can be penalized for their error. Common material that falls under NDA are blueprints, passcodes, maps of certain facilities, logs of personal movements (such as military activity) and private documents. Most anything classified as 'classified,' 'secret,' or 'top secret' is not disclosed to the public. Some individuals have special privilages in courts of law to refrain from disclosing certain information. For example, journalists are granted the privilage to keep their sources secret, under the idea that sources would no longer come forward with information if journalists were made the law's patsy.
Keeping material secret is not always a sinister action, or even a threatening one; nearly every corporation will try and protect the design of their products from competitors, and covering the movements of military units or other government individuals can save lives. Corporate and government intelligence (in this case you can just call them spies) may actively work to discover undisclosed information.
However, there is a common thought thread to many conspiracy theorists— "If they are hiding something from me... it must be because I would rebel if I knew! And if I would rebel if I knew... they must be taking advantage of me! And if they are taking advantage of me... they must be evil and plotting my/our/the world's demise! And if they're plotting the world's demise, they must be communists/jews/nazis/aliens/lizard people! Some sinister force is exploiting me for money and power! I must fight back and no one realizes it! I must be the most special and intelligent person in the world if I can see it and no one else can!"
In other words, secrets are scary. In the circles of conspiracy theorists and UFO nuts, disclosure of mythical documents or information would be downfall of the alleged government cover-up of UFOs being alien spacecraft/there's life on Mars/whatever the particular user of the term believes in. There seem to be at least two different lines of thought about how this can happen.
The first one treats the Disclosure approximately as some Christians treat the Rapture - as something that may happen at any moment, due to forces higher than them. The Powers That Be are supposed to be feeding the sheeple the truth bit by bit, preparing them for the Disclosure. Thus, various incidents and/or news events are interpreted as deliberate leaks, leading towards this goal. Examples can be found on conspiracy forums every time when there is a news announcement about research on life in outer space, resulting in triumphant shouts of "Disclosure imminent!" or the more cautious "Disclosure?". In reality, disclosure of government or corporate secrets is either a quiet declassification of materials that are no longer priority to protect, or a crime (often the breaking of an NDA)that must be perpetrated by an actual person. Either way, it's not some grand worldwide conspiracy reveal like a twist in an action movie.
The other aspect is simply "fighting for disclosure" - the belief that the government can be pressured into releasing "the Truth" about UFOs/aliens/whatever. This is the stated goal of The Disclosure Project. This likely will be somewhat difficult when the government doesn't actually have a truth to reveal about the ridiculous woo of the week.