A more thorough explanation:

The clearly-erroneous standard is a legal term used to describe the standard of review that an appellate court usually applies when reviewing a trial court's treatment of factual issues. Under this standard, a judgment will be upheld unless the appellate court is left with the firm conviction that an error has been committed.

For example, if a trial court finds that a defendant is guilty of a crime based on the evidence presented at trial, the appellate court will apply the clearly-erroneous standard when reviewing that finding. The appellate court will only overturn the trial court's finding if it is convinced that the trial court made a mistake in its interpretation of the evidence.

Another example would be in a civil case where a trial court finds that a plaintiff is entitled to damages based on the evidence presented at trial. The appellate court will apply the clearly-erroneous standard when reviewing that finding. The appellate court will only overturn the trial court's finding if it is convinced that the trial court made a mistake in its interpretation of the evidence.

These examples illustrate how the clearly-erroneous standard is used to ensure that trial court judgments are not overturned unless there is a clear error in the interpretation of the evidence presented at trial.