A writ of prohibition is an order to a court to cease trying a case. The writ also covers the parties involved in the case, ordering them to cease their activities. There are several reasons why a writ of prohibition might be issued. Such writs are relatively rare, as legal situations rarely proceed to the point that a writ of prohibition would be required.
The writ is usually issued by a higher court which has authority over a lower one. One reason for a court to issue a writ of prohibition is if it believes that a lower court is acting outside its jurisdiction. When courts exceed their jurisdiction, they do not actually have authority over the matter at hand and it must be tried in another court of law in order for the results to have legal standing. Courts usually try to avoid hearing cases which are not in their jurisdiction, thus eliminating any reason to have a writ of prohibition issued to get them to stop handling given cases.
Another reason for such a writ is a situation in which a court is not operating under the normal rules of procedure. Just as with courts which exceed jurisdiction, not following procedure jeopardizes the validity of the decision reached by the court. People involved in the trial can later challenge the outcome on the grounds that the trial was not conducted properly and thus that the decision is not legally binding.
If a decision will defeat a legal right, a higher court may issue a writ of prohibition. All of these situations are relatively rare and circumstances in which a writ of prohibition would be necessary or appropriate are consequently very unusual. When a court issues such a writ, it must provide information about why it is being issued, and this information may be used to argue against the order to stop on the grounds that the writ was issued in error.
The legal system is deliberately constructed in layers to protect the rights of citizens and the integrity of the legal system itself. The higher in the legal system a case progresses, the more authority a court has. This allows people to step through multiple layers of authority as they prosecute or appeal a case, with scrutiny from different angles at every layer to confirm that the case is handled appropriately. Higher courts can issue other orders to lower courts compelling them to take different actions if the judge of a higher court has reason to believe that a lower court is not acting appropriately.