These are intended to facilitate and assess understanding of the reading materials. Reading reflections should be within one page (roughly within 600 words if you are using 12pt font). You won’t be penalized if you write more, but being succinct is another great writing skill which you should aim to cultivate in this course.
You do not need to summarize the full paper, but you need to reflect on what additional questions the work enables. Does this help you think about your next big project? What will that be? What other questions the paper makes you think? What else the paper is not answering or is concerning or is just intriguing?
Again this is an individual assignment and work submitted should be written solely by you. Most importantly, a reader while glancing at your reflection should be able to easily spot these questions. So use bold, italics, bullet points or other means of highlighting them. Here is a great example of a reflection written by my colleague, Prof. Kurt Luther.