This is a Grand Jury so it's just looking at whether there is enough evidence to bring a criminal case. The burden of proof is much lower, there is no judge, no case for the defence and evidence rules are relaxed.
This is it in a nutshell. All a GJ does is review the evidence and hear witnesses and then decide if there is enough to bring a case to trial. I think in this case what may be the point is that anyone can be subpoenaed and questioned without their attorney present. Questioning can be very broad, much broader than in a criminal trial where someone's attorney would probably object to the line of questioning. A witness can still plead the 5th and can ask for immunity in exchange for testimony. GJs are sometimes used to question someone about their own activities who has refused questioning elsewhere in hopes of getting them to 'spill' about a third party (the actual real subject of the investigation) in exchange for immunity. So it will be interesting to see exactly who gets subpoenaed to testify before the GJ.
I'm sure the judge screening the applicants will be very careful to balance the jury as best he/she can to make it as impartial as possible.
The wiki is pretty good at explaining this. And there is a specific section on Federal Grand Juries.
en.wikipedia.org/wiki/Grand_juries_in_the_United_States
I don't remember the show, but someone said that if Scrotus gets subpoenaed his attorneys need to give him the 5th amendment written on a 3 x 5 index card and tell him that all he is to do is read it out loud after every question and say nothing else. I say that's a physical impossibility for him.