A retrial is a second trial for the same charge or charges. Retrials are relatively rare in the US as most matters are settled either in the first trial or out of court. As it is illegal to try a person twice for the same crime, retrials can only happen if in the first trial the jury does not reach a verdict of either guilty or not guilty. Retrials therefore can happen in the case of mistrials, or if the jury hangs.
In the case of a retrial, the jury is not informed that they are the second group to hear the case. If they do find out that there was a previous trial with a jury that hung, it is grounds for mistrial.