Exactly What It Says
Persons the government feels are to dangerous to be let out of custody if found innocent at trial.
Under federal and state laws persons at trial have rights to examine all evidence the state has against them in open court. The federal government is obviously loathe to release such information for various reasons, one of which is that it would become public and part of court (public) records, thus the state would have one if not two hands tied behind it's back.
With that sort of odds, the persons would likely be found innocent, and the natural course of law means they must be released.
Under the old regime of European monarchs, a person could be detained at "the king's pleasure" for any reason for any length of time. Indeed a reason often did not have to be given or it could be kept "secret. Nor could one demand access to papers or evidence to prove one's innocence.
The founding fathers of the United States sought to end and break away from this practice, hence the Consitution gives rights to individuals and puts checks and limits on goverment powers. This is what gave the previous administration and the current one so many fits. The United States is supposed to be the one country in the world where one cannot be picked up off the streets, taken to a secret location and kept there with limited or no communication with the outside world, and or no access to legal representation along with a fair and open trial.