A closed proceeding in which citizens decide whether there is enough evidence to charge someone - not whether they are guilty.
A grand jury is a body of citizens that decides whether a prosecution may be brought. It is not the jury that decides a case. It hears the prosecution's evidence, in private, and votes on whether the standard for charging has been met; if it has, it returns an indictment, the formal document setting out the charges.
Almost everything about the proceeding differs from a trial, and the differences are the reason indictment rates are high. The defence is generally not present and does not cross-examine. There is no judge in the room presiding over the presentation. The standard is probable cause, not proof beyond a reasonable doubt. The rules of evidence largely do not apply, so material that could never be put before a trial jury may be heard. And the proceeding is secret, with disclosure of what happened restricted by rule.
Being called before a grand jury does not mean a person is a suspect. Witnesses are routinely subpoenaed to give evidence or produce documents, and a witness retains the privilege against self-incrimination and may decline to answer questions on that basis. A witness is normally not permitted to have a lawyer beside them in the room - the usual practice is that they may leave to consult one - which makes advice before attending, rather than during, the useful step.
Grand juries are not universal in the United States. The federal system requires an indictment for serious offences, but the Supreme Court has never applied that requirement to the states, and states are divided: some require indictment for felonies, and many permit prosecutors to charge instead by information, with a judge testing the evidence at a preliminary hearing. Which route applies determines whether there is any early public hearing on the evidence at all.
A grand jury subpoena is a reason to speak to a lawyer before the return date, whoever the recipient is: the distinction between a witness, a subject and a target is one the prosecutor draws and does not always announce, and it is the first thing to establish. A subpoena for documents raises separate questions about scope and about privileged material, and complying by handing over everything asked for is not always the correct response. Anyone who has been told they are a target is in a different position from a witness and should not attend to "clear things up" without advice - grand jury testimony is given under oath and stands for the life of the case.
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