It means the indictment has been handed down, but the DA does not want the information made public until a particular date passes or particular conditions are met. After the date passes or the conditions are met, it becomes public information.
Likely you will not know anything: who, what, when, where, or why. In other words, the fact of the indictment may be released, but not who has been indicted, or what they have been indicted for.
Sealed indictments are normally used when the grand jury has indicted someone for a felony on the basis of the evidence placed before it, but the person in question is not yet in custody. They are sealed to prevent giving forewarning to the indicted person … presumably to keep him/her from hiding or fleeing before apprehension.
They may also be used when the person in question is a public figure, who may have the political influence to sway the legal system if forewarned by the indictment becoming public before the appropriate time. (E.g., grand juries investigating the possible illegal behavior of political bosses would hand down sealed indictments, so that the grand jury session could be terminated and the jurors safely located out of the boss’s area of influence before he and/or his leading henchmen were arrested based on the indictments.)