A target witness is a witness who is believed, by a grand jury, to possess crucial information, and is someone who the grand jury would like to indict.
If a target witness is indicted, then he or she can be called to the witness stand to testify in a trial. Target witnesses have the ability to refuse to answer self-incriminating questions before testimony.
The reason why grand juries often want to indict a target witness is because they can possess specific information relevant to the trial, even if they are not a defendant. For example, if a defendant is accused of robbing a bank, the grand jury may seek to indict the defendant's brother as a target witness. This could be because the defendant lives with his brother, and the grand jury has reason to believe that the defendant's brother knows crucial information about the crime.