A reversible error is any error that a judge makes in applying the law to a given case. It is not an error that the jury makes in coming to the "wrong" decision in a case.
There are many possible examples of reversible errors. For example, a judge may allow a piece of evidence to be admitted when that evidence was obtained illegally. In such a case, an appellate court could reverse the decision on the basis of that error. As another example, a judge might allow the sexual history of a rape victime to be brought up in a trial in violation of a rape shield law. Because this testimony should not have been allowed, this would be a reversible error.
Essentially any error in the application of law that could have a major impact on the jury's decision can be a reversible error.