An appeal was a procedure in English law to bring about a prosecution by a private party of an individual accused of a heinous crime.
[1]: 311 "[The term 'appeal'] is derived from the French, 'appeller' ... which signifies to call upon, summon or challenge one; and not ... the same as the ordinary sense of 'appeal' [from an adverse litigative result].
It probably descended from the system of weregild where private pecuniary satisfaction was paid to the party injured, or his relations, to expiate an enormous offense.
An acquittal following an appeal prevented any further prosecution for the same offence (the doctrine of double jeopardy), just as if the proceedings had been brought by the Crown (by indictment).
[1]: 314 The appellor would be punished with one year's imprisonment, and pay a fine to the king, besides restitution of damages to the appellee.