Grand juries typically fulfill both an investigative and charging function.
1) When it comes to investigating, the grand jury is granted the ability to subpoena documents and witnesses.
In a complicated and long-term criminal investigation, federal prosecutors will go to the grand jury to ask to issue subpoenas for the production of documents or records they deem necessary for the case. Also, if prosecutors want to force witnesses to testify under oath, they go to the grand jury to subpoena the person to appear.
2) For the charging function, a grand jury must determine that probable cause exists to believe that an individual has committed a crime before the person can be charged for the crime.
Grand juries make the preliminary decision of whether or not to officially accuse a criminal defendant and compel him or her to stand trial. Grand jury indictments are required for all federal felonies.
"At their core, grand juries are about finding probable cause that a crime has been committed," says Danny Cevallos, CNN legal analyst.
Unlike a typical trial, grand jury proceedings are not public and there is no cross-examination or presentation of the defense case. The jurors may ask questions and witnesses may not have lawyers present during the questioning.
The grand jury will determine whether criminal charges should be brought at the end of the thorough investigation.
What's the significance of Mueller's grand jury activities?
Issuing grand jury subpoenas doesn't signal that indictments are imminent or that Mueller has concluded there has been wrongdoing. The move is often simply a tool to allow a prosecutor to gather evidence or testimony.
"What it means is that Mueller can now take sworn testimony in front of the grand jury and start subpoenaing people, asking people to testify, and of course if he gets enough evidence, he can now ask the grand jury for an indictment of people," says Jeffrey Toobin, CNN senior legal analyst.
It suggests Mueller is at least taking a preliminary next step -- but it doesn't mean there will be an indictment.
"It is certainly a significant step that shows that he means business. It shows that he's doing a serious criminal investigation, which he thinks should proceed. But it doesn't mean that criminal charges are imminent nor does it mean criminal charges will ever happen," Toobin said.
How long can grand juries take?
Grand jury investigations can be long processes, lasting for months or years. The grand jury continues as the prosecutors chase evidence and leads to compile a case.
The term grand jury is derived from the fact that it usually has a greater number of jurors than a trial jury. A grand jury typically consist of anywhere from 16 to 23 people, and the jurors may have to sit for months at a time.
Grand jury proceedings remain secret.
How has Trump's team reacted?
Ty Cobb, special counsel to the President, said he wasn't aware that Mueller had started using a new grand jury.
"Grand jury matters are typically secret," Mr. Cobb said. "The White House favors anything that accelerates the conclusion of his work fairly... The White House is committed to fully cooperating with Mr. Mueller."