Due process is one of the cornerstones of American jurisprudence, and the concept is rooted in the basic notion of fairness. In the context of a court proceeding, due process generally requires that all parties be given advance notice of a hearing or trial and an opportunity to be heard by the court before a decision is made by the judge.
In order to insure fairness to all parties, ex parte communications with the judge are generally prohibited, with certain exceptions. Obviously if the judge only hears one side of the case, he or she will not be getting the whole picture, and might unduly favor the litigant engaging in the ex parte communication. Literally, the term means “on one side only”, without the other side having the opportunity to be heard.
One exception to the rule against ex parte communications involves applications for injunctions, where emergency relief might be needed and where there is insufficient time to notify all parties before issuing an injunction. However, emergency injunctions (often called “temporary restraining orders”) are typically of short duration, with the requirement that the court hold a hearing on the injunction, usually within five days, in order to provide all parties with notice and an opportunity to be heard.
— BBC&B