An open-ended question asks for the answerer to provide information, so you are being asked to write questions about The Pearl that will require something like short essay answers. Your questions are apparently supposed to focus on the themes presented in the novel.
The themes in the novel are presented as contrasts between two extremes - good and evil, knowledge and ignorance, the individual and society as a whole. So, your questions need to be written to ask someone answering them to give information about something in the story that relates to one of these themes. Open-ended questions frequently start with words like what, why, or how because answering those types of questions demands the providing of information.
For example, one of your questions might ask "How does the music in Kino's mind help or hurt him?"