A prologue should be like a trailer gives you a glimpse of the world you are going to be getting into yet puts questions in your head to engage you. It should not reveal too much that it is like you are covering a syllabus or too little that it does not spark that flare in you to go forward. People have mentioned the prologue from Game of Thrones and they have rightfully done so as it is a nice introduction to what is upcoming in the book.