The GJ is briefed by the prosecutor about the case. The foreman of the grand jury swears in each testifier and they give their testimony. (Usually, police or detectives, witnesses (from either side mind you), etc.) Then the grand jury asks the questions of each person that testifies. And they can pretty much ask anything they want that they feel is relevant to determining whether or not there is enough evidence to indict. So, as a GJ you are kind of looking at it from the defense side, because you're trying to make sure the prosecution has enough evidence to indict. Does that make sense? So, let's say the prosecution says Terri's phone pings didn't match where she said she was. You are provided documents that will show that and be able to question the person(s) who provide that information and ask them how that works, etc. Then you can ask them, can that be wrong, etc., can a ping actually have bounced from here to here, etc. And if they say something to the effect of yes, but that would be unusual. Then you as a GJ ask yourself how good then, is that evidence. And you might ask could it have been someone else using her phone? That kind of thing. You make notes on every testimony. (You can be re-convened on a case, or have to testify in court as to what you heard in the GJ, though it never happend to any of us.) They keep ALL notes under lock and key.
Hope this helps explain the process in lay terms.
Hope this helps explain the process in lay terms.