As special counsel Robert Mueller drives headlines in the Trump-Russia probe, a grand jury is playing a key role behind the scenes. This group of anonymous Americans is working with Mr. Mueller to issue subpoenas, hear testimony and examine evidence.

Ultimately, it will be up to the grand jury to decide if there is probable cause to issue an indictment. And because the grand jury is independent, President Trump can’t shut it down, even if he decides to fire Mr. Mueller.

The...