Witness preparation refers to a process that takes place before a witness is set to give testimony in court, and is aimed at preparing the witness to better present their side of the argument and make it as clear as possible what side they're taking in it. It should be noted that while witness preparation is conducted by lawyers, there are some strict limitations over what they could discuss with the witness before the case - for example, lawyers are prohibited from instructing the witness to give any specific testimony or altering their views in any way - instead, they are only allowed to coach the witness in a way that will clear out any misunderstandings and dubious statements from their testimony.
Witness preparation is common for both high- and low-profile cases, and it's a very important aspect of any legal proceeding. Proper witness preparation can play a deciding role in the outcome of a case, and some lawyers are specifically trained to better prepare their witnesses and coach them appropriately.
Even witnesses who're experienced in testifying in court are often subjected to witness preparation, especially when their testimony is of vital importance to the success of the case. The process may be conducted in a specially prepared room that mimics the appearance of an actual court room, to ensure that the witness is as prepared as they should be, and acquainted with the specifics of a courthouse environment.