An NSL is a type of subpoena used by the United States federal government to request information pertinent to matters of national security. This communication is typically accompanied by a nondisclosure agreement that prohibits the recipient from making the request known to the public.
However, the enactment of the USA Freedom Act introduced a stipulation that forces the FBI to periodically assess whether nondisclosure requirements associated with particular NSLs are still warranted. If there’s no reason to keep the request under wraps, the gag order is lifted and the recipient can make the information known if they see fit