In the law of evidence, certain subject matters are privileged, and can not be inquired into in any way. Such privileged information is not subject to disclosure or discovery and cannot be asked about in testimony. Usually, privileges exist not because of a fear that information provided will be inaccurate, but because there are public policy reasons the information should not be disclosed.
For more about specific privileges, see privilege against self-incrimination, spousal communications privilege and attorney-client privilege.