An indictment is evidence, including witness testimony brought before a Grand Jury. Presented to a panel of up to 23 jurors, they decide whether the Prosecutors should bring charges against the accused based on the evidence. Indictments are sealed because a)The accused may be part of a Conspiracy involving others who haven't yet been undicted b) Prosecution has not yet formally put the case together for trial and doesn't want to alert the subject or c) to apply for additional warrants or summons'. It's the first step of getting to trial.