>What does it mean if a grand jury is impanelled?
Unlike trial juries, grand juries don't weigh in on a defendant's guilt or innocence. Instead, they convene before any charges have been filed in order to determine whether charges or an indictment should be filed. Prosecutors decide whether or not to impanel a grand jury, and they usually only do so when serious felony charges are a possibility.
Like a trial jury, a grand jury is presented with evidence, exhibits, and testimony from witnesses about matters pertinent the investigation. But the setting in which this unfolds is different. Trial juries meet with a judge, lawyers, the defendant, and the plaintiffs present, and the proceedings are generally open to the public. Grand juries, on the other hand, meet in secret, and aside from the prosecutor who pressed the charges, nobody else is usually present. Crucially, the potential defendants — that is, the person or people against whom charges might be pressed — are not allowed to attend grand jury proceedings.
From here -
https://www.bustle.com/p/what-does-impaneling-a-grand-jury-mean-robert-mueller-is-pushing-the-russia-investigation-forward-74432