A more thorough explanation:

A Grand Jury witness is a person who is asked to appear before a Grand Jury because they may have information or knowledge about a matter that the Grand Jury is investigating. The Grand Jury proceedings are conducted in secret, and only the witness, attorneys, an interpreter (if needed), and a court reporter are present.

The witness is required to testify and answer questions about the information under consideration by the Grand Jury. They must answer all questions truthfully, except where the privilege against self-incrimination applies. If a witness provides false answers, they could be prosecuted for perjury.

Anything a witness says that could incriminate themselves may be used against them by the Grand Jury or later in court. However, a witness may consult with an attorney before testifying, and they may have an attorney outside the Grand Jury room.

Grand Jury witnesses are entitled to the same witness fees and travel expenses as all other witnesses. They receive a $40 witness fee for each day they are required to be in court or attend a pretrial interview, including travel days. The government reimburses all legitimate travel expenses related to the testimony.

John was called to appear before a Grand Jury because he witnessed a crime. During his appearance, he was asked questions about what he saw and heard. He answered truthfully and provided the information he knew. Since he did not incriminate himself, he was not at risk of being prosecuted for perjury.

Another example could be a witness who is asked to testify about a company's financial records. They would be required to answer all questions truthfully and provide any information they have about the matter under investigation.

These examples illustrate how a Grand Jury witness is someone who has information or knowledge about a matter under investigation and is required to testify truthfully before the Grand Jury.