A court witness is a witness who gives a testimony during a trial at the request of the court. An example of a court witness would be a surgeon who gives a testimony during a medical case at the request of a judge. In this case, the surgeon could help to explain medical procedures which are relevant to the case
Sometimes during a trial, the court calls its own witnesses. It does this primarily when there is an issue that needs further explanation; this is often done if the subject matter at hand is complicated. A court witness can help the jury understand what is going on. The testimonies of witnesses are often critical to the outcome of the trial. This is why the court calls its own witnesses if it believes it is necessary to clarify something. Court witnesses can also help the court in general understand the subject at hand.