First understand that a Grand Jury is a pool of people selected from the same pool of potential jurors for trial juries. However, there is no selection process like in a trial. A prosecutor will make a request to get a grand jury in place. Most of the time it is pretty routine and a grand jury will be sitting at all times to hear the day to day criminal matters that a prosecutor wishes to pursue. The grand jury is supposed to act as a safeguard to make sure that a defendant is only brought to answer for legitimate crimes with sufficient evidence to warrant a trial.
In this case, you have a special prosecutor requesting the formation of a grand jury. Notices (summonses) go out to about 35 people who live in the jurisdiction that the potential criminal case is being investigated in. Between 16 and 23 will be selected to serve. Only really severe hardship will keep you from serving. They will be sworn in. That is what being impaneled means. A grand jury has been put together to hear evidence that the special prosecutor has assembled. They will eventually decide if indictments should be issued based on that evidence. It does not have to be unanimous. There is no deliberation, but they can ask questions of both witnesses and the prosecutor. They do not necessarily meet every day to hear evidence. They can be called in to hear evidence multiple times. Multiple days of the week. It can be for a period of many months, until the prosecutor has completed his investigation. Only the prosecutor's evidence and witnesses are presented, although the prosecutor may bring in many witnesses that cover a broad range of facts, events and circumstances-mostly to get some of the witnesses to be locked in to their stories (testimony is under oath and you can be prosecuted for perjury if you lie or withhold evidence). They can't change their stories later and witnesses can be pressured to make deals in return for immunity or lesser charges. It is not a court case like you see on TV. There is no judge present, only the prosecutor and witnesses. The proceedings are supposed to remain secret.
At the end of the process, the prosecutor will request the grand jury to return an indictment(s) against any defendant(s) that the prosecutor feels he has established probable cause to stand trial. He will explain the law to the grand jury. He will state what charges he wishes to bring and ask them to vote to indict. If at least 12 of the grand jurors vote to indict and they state which charges they are indicting on, they will return what is known as a true bill and charges will be brought against the defendant. It only means that they feel the prosecutor has sufficient evidence to make somebody stand trial. Famously, it has been said that a good prosecutor can get a grand jury to indict a ham sandwich. Having presented cases to a grand jury, that is not quite true, but it is pretty easy to get an indictment as long as you have some evidence. (the prosecutor does not have to present everything he has to the grand jury and the rules of evidence are not the same as in a trial)
The grand jury can hear evidence about any wrongdoing the prosecutor has uncovered. Any matter that is within the jurisdiction of a federal court. Any ancillary matters that come up during the prosecutor's investigation or during the course of the gathering of testimony in front of the grand jury. The grand jury can actually request additional evidence or witnesses, but the prosecutor is not obligated to present it. They are generally very deferential to the prosecutor, but not always. It can take a period of months or even years, but that is very rare to go longer than 6 months. Mueller has probably been investigating Russia connections since the start but has only recently developed enough information to start getting people under oath, on the record and testifying in a manner that is designed to build a case that individuals should be indicted and held to stand trial. He may also have multiple grand juries hearing different matters at the same time.
I hope that answers your questions.
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