Grand juries have the power to conduct investigations. They have very broad subpoena powers to obtain documents and tangible items. Often, grand juries subpoena people appear before them to testify.
If you are a witness and refuse to testify, you can be held in contempt and placed in jail. If you are the target of the grand jury investigation, you have the right to remain silent.
Grand jury proceedings are secret and the testimony heard before them is sealed.
If you are currently under grand jury investigation, it is imperative that you immediately contact an attorney. A skilled and seasoned attorney may be able to help you avoid being indicted.