A grand jury is a group of people that are selected and sworn in by a court, just like jurors that are chosen to serve on a trial jury. In fact, the grand jurors are usually chosen from the same pool of people that provide trial jurors.
When the grand jury meets, a prosecutor or law enforcement agent will present a case, and the jurors will decide whether there is probable cause. If there is probable cause, they return an indictment (also called a “true bill”). If they find that there is not probable cause, a “no bill” is issued and the case ends.