A more thorough explanation:

The appeal of felony was a historical legal procedure in which a person accused another of a crime, demanded proof of innocence by wager of battle, or informed against an accomplice. It was also known as appellum de felonia.

For example, if someone was accused of a serious crime like murder, the accused could demand a trial by combat to prove their innocence. Alternatively, someone who was involved in a crime as an accomplice could inform on their partner in exchange for a lighter sentence.

These examples illustrate how the appeal of felony was used in medieval times as a way to determine guilt or innocence and to bring criminals to justice.