“No person shall be held to answer for a capital, or otherwise infamous crime, unless on a presentment or indictment of a Grand Jury, except in cases arising in the land or naval forces, or in the Militia, when in actual service in time of War or public danger…”
Being prosecuted for a serious crime is a big deal, even if you are ultimately found to be not guilty. It is stressful, expensive, distracting and damaging to your reputation. The 5th Amendment’s requirement of a grand jury is designed to prevent the government from putting you on trial unless it has at least some good evidence against you.
If a prosecutor in a federal trial has enough evidence to convince a group of your fellow citizens that you might have committed an “infamous” crime, then those citizens, the “grand jury,” will issue a formal accusation, called an indictment. If the prosecutor fails to convince the grand jury, then the case against you cannot proceed.
Grand juries can also serve more generally as a check on governmental power. In the years before America’s Revolution, grand juries sometimes stymied the government’s attempts to bring a case to trial not only for insufficient evidence, but simply because the colonists objected to certain unpopular laws with which a person was being charged.