What is a standard of review? It is the rule that a higher court uses to determine whether or not to correct something that happened in a lower court. Most appeals are appeals from trial courts. The two most common standards of review are:(1) abuse of discretion and (2) de novo (A review by the de novo standard may also be called a review by question of law.).
A judge can make many decisions at trial that observers might disagree with, but they might not be reversible error. A judge may find that some evidence is repetitive of earlier evidence and not let it in, while another judge would let it in. For one of the parties to complain about a decision about that is not likely to move an appeals court. The higher court doesn't want to "Monday morning quarterback" trial judges about decisions like that. The trial judge has the litigants and their counsel before the court, and is more likely to make a good decision than appellate justices reading a cold record a year later.
Some decisions appellate courts expect trial courts to get right such as whether to grant a summary judgment or to leave a question out of a jury charge. Appeals courts don't defer to the trial judges about such things. The appeals court will second-guess trial judges about such important matters.