Insiders are members of secret societies, military projects, or government agencies who are privy to non-public information. For various reasons, insiders may leak some of this information to the public. Often they do it anonymously, perhaps through third party contacts on the outside who can vouch for their identity but keep it confidential while relaying the information.
They may also speak openly without hiding their identity, but then tend to be careful about not revealing more than they are allowed. Some self-claimed insiders are casually upfront and detailed, seemingly holding nothing back.