A reflection paper is a type of paper that students may be assigned to complete. I have mostly encountered reflection papers in university, especially in graduate school, but the concept is fairly simple, and is probably used widely, although how and when a reflection paper is used varies widely amongst teachers.
A reflection paper is basically a chance for a student to write down their thoughts and feelings about something that they have been assigned to read or something that was discussed in class. Reflection papers usually have a short cycle compared to other types of papers: they would usually be turned in within a few days of the subject matter having been discussed and read. The length of a reflection paper is also fairly short: usually only one to several pages. Although sometimes reflection papers are required to have formal presentation and include references, they usually are meant to reflect the student's thoughts in a casual way. The reflection paper is much less complicated than either a research paper or an essay, and is only marginally more structured than something such as a free write.
I often find writing reflection papers myself to be ridiculously easy, but part of that may be that I am usually able to come up with several paragraphs of my feelings on any given topic (thus my success on this website). For people less inclined to put their thoughts in writing, even a two page reflection paper may still be a task. However, quite apart from their difficulty level, reflection papers are probably useful. I think the idea behind a reflection paper is that if a student knows that they are going to be writing something, they will interact with a text or discussion more, planning on ways to turn that receptive experience into a generative one. At the very least, it is a way for the teacher to make sure the student is at least paying a cursory glance to their text.
With that being said, I have to admit that many of the reflection papers I have written for graduate school wouldn't even match the quality of my daylogs here. All those jokes about Piled Higher and Deeper exist for a reason.