A more thorough explanation:

The clearly erroneous test is a standard used by higher courts when reviewing a decision made by a lower court. This test is established by Rule 52(a) of the Federal Rules Civil Procedure. According to this rule, a judge's factual findings in a non-jury trial are considered valid unless they are "clearly erroneous." The reviewing court must also consider the lower court's ability to assess the credibility of witnesses.

For example, if a lower court finds that a defendant is guilty of a crime based on the evidence presented at trial, the higher court will use the clearly erroneous test to determine if the lower court's decision was correct. If the higher court finds that the lower court's decision was not supported by substantial, credible evidence, it may overturn the decision.

The Supreme Court has defined "clearly erroneous" as a finding that, although supported by some evidence, leaves the reviewing court with a "definite and firm conviction" that a mistake has been made. Essentially, the higher court must determine that the lower court's decision was not based on credible evidence in the record.