A sealed verdict is a written document that is placed into an envelope and sealed. On the document is written the jury's verdict for a particular case.
Juries often arrive at their verdicts when the court is not convened, thus the verdict must be sealed and then delivered at the next appropriate time.
A sealed verdict must be opened and read at a trial for it to become legitimate. After it is read, the jurors can be dismissed, and the judge can conduct the sentencing for the criminal.
Many trials do not require sealed verdicts. If the jury arrives at a verdict while the court is still convened, then the verdict can simply be given to the court. Sometimes, however, the jury has to wait (if, for example, a particular judge is not present).