A Non-Disclosure Agreement (NDA) is a legal document obliging the signatory to refrain from discussing or sharing sensitive information without specific permission. For fans this usually applies when working with LEGO on a particular project.
The original members of the LEGO Ambassadors program for example had to sign NDAs as did the participants in the LEGO Universe and Hobby Train projects. In each case the participants were being shown items long before the general public. The NDA would have imposed legal sanctions if any of those items had been shared with clone brands or other competitors.