An appeal is not a retrial or a new trial of the case. The appeals courts do not usually consider new witnesses or new evidence. Appeals in either civil or criminal cases are usually based on arguments that there were errors in the trial’s procedure or errors in the judge's interpretation of the law.
The appellate court determines whether errors occurred in applying the law at the lower court level. It generally will reverse a trial court only for an error of law. Not every error of law, however, is cause for a reversal. Some are harmless errors that did not prejudice the rights of the parties to a fair trial. For example, in a criminal case a higher court may conclude that the trial judge gave a legally improper instruction to the jury, but if the mistake were minor and in the opinion of the appellate court had no bearing on the jury's finding, the appellate court may hold it a harmless error and let a guilty verdict stand.
However, an error of law, such as admitting improper evidence, may be determined to be harmful and therefore reversible error.
https://www.americanbar.org/groups/...ducation_network/how_courts_work/appeals.html
