Above BBM
According to the ABA:
Why are grand jury proceedings secret?
Rule 6(e) of the Federal Rules of Criminal Procedure provide that the prosecutor, grand jurors, and the grand jury stenographer are prohibited from disclosing what happened before the grand jury, unless ordered to do so in a judicial proceeding. Secrecy was originally designed to protect the grand jurors from improper pressures. The modern justifications are to prevent the escape of people whose indictment may be contemplated, to ensure that the grand jury is free to deliberate without outside pressure, to prevent subornation of perjury or witness tampering prior to a subsequent trial, to encourage people with information about a crime to speak freely, and to protect the innocent accused from disclosure of the fact that he or she was under investigation.
However,
Why can a grand jury witness talk about his or her testimony?
In the federal courts, the witness is not sworn to secrecy, and may disclose whatever he or she wishes to whomever he or she wishes. The witness exemption was adopted in part because it was thought that requiring witness secrecy was unrealistic and unenforceable, and in part to allow the witness to rebut rumors concerning his or her testimony. There is a basic revulsion in the United States about secret testimony.