Comment on cases that are actively before a court of law which amounts to contempt of court and prejudicial to fair trial. Such comment may include criticism of the prosecution and damaging statements about the accused.
There is usually little risk of contempt in civil cases. In criminal cases which may go before a jury the risk is much more serious. The sub judice period ends once the jury has passed a "not guilty" verdict, the prosecution is dropped, or a defendant is sentenced.
The phrase is often used by spokespersons for government departments, local authorities and other bodies in order to avoid answering embarrassing questions.