Courts occasionally appoint special masters – typically retired judges or lawyers – to decide if materials seized by subpoenas or search warrants are protected by attorney-client privilege, and therefore exempt from use in court. Unusually, Cannon also authorized the special master in this case to look for documents protected by “executive privilege”.
Executive privilege is the controversial legal doctrine that US presidents, and their staffs, can refuse certain legal demands if they impair national security or the functions of the executive branch.
What makes the situation even more bizarre is that Trump, a former president, is invoking executive privilege against the current executive branch of the US government.